The  Fundamentals 

of 

Photography 

and 
Other  things 

'By  C.  E.  K.  Mees,  D.Sc. 


Eastman  Kodak  Company 

Rochester,  N.  Y. 


H   CENT^ 


/ 


uL 


ti^yfc^  -ry^f //Cl,    }u  cryx^ 


The  Fundamentals 

of 

Photography 


'By  C.  E.  K.  Mees,  D.Sc. 


Eastman  Kodak  Company 

Rochester,  N.  Y. 
1920 


THE  GETTY  CENT  l 
LIBRARY 


PREFACE 

WHILE  a  knowledge  of  the  theory  of  photography  is  by 
no  means  essential  for  success  in  the  making  of  pic- 
tures, most  photographers  must  have  felt  a  curiosity  as  to 
the  scientific  foundations  of  the  art  and  have  wished  to 
know  more  of  the  materials  which  they  use,  and  of  the  re- 
actions which  those  materials  undergo  when  exposed  to  light 
and  when  treated  with  the  chemical  baths  by  which  the 
finished  result  is  obtained.  This  book  has  been  written 
with  the  object  of  providing  an  elementary  account  of  the 
theoretical  foundations  of  photography,  in  language  which 
can  be  followed  by  readers  without  any  specialized  scien- 
tific training.  It  is  hoped  that  it  will  interest  photogra- 
phers in  the  scientific  side  of  their  work  and  aid  them 
in  getting,  through  attention  to  the  technical  manipulation 
of  their  materials,  the  best  results  which  can  be  obtained. 

Rochester,  N.  Y. 
August,  1920. 


CONTENTS 


CHAP. 

page 

Preface          

3 

I. 

The  Beginnings  of  Photography 

7 

II. 

13 

III. 

About  Lenses 

19 

IV. 

The  Light  Sensitive  Materials  Used  in 

Photography      

33 

V. 

Development          

39 

VI. 

The  Structure  of  the  Developed  Image 

48 

VII. 

The  Reproduction  of  Light  and  Shade  in 

Photography      

57 

VIII. 

Printing 

69 

IX. 

The  Finishing  of  the  Negative   . 

85 

X. 

Halation 

97 

XI. 

Orthochromatic  Photography 

101 

Digitized  by  the  Internet  Archive 

in  2011  with  funding  from 

Research  Library,  The  Getty  Research  Institute 


http://www.archive.org/details/fundamentalsofphOOmees 


THE  FUNDAMENTALS  OF 
PHOTOGRAPHY 

CHAPTER  I. 

THE  BEGINNINGS  OF  PHOTOGRAPHY. 

THE  first  person  to  notice  that  chloride  of  silver  was 
darkened  by  light  seems  to  have  been  J.  H.  Schulze, 
who  made  the  discovery  in  1732.  We  must,  therefore, 
regard  Schulze  as  the  father  of  photography.  In  1 737  Hellot 
in  Paris,  was  trying  to  make  sympathetic  inks,  that  is,  inks 
that  would  be  invisible  when  put  on  paper  but  which  could 
be  made  visible  afterwards.  He  found  that  if  he  wrote  on 
paper  with  a  solution  of  silver  nitrate,  the  writing  would  not 
be  visible  until  the  paper  was  exposed  to  light,  at  which 
time  it  would  turn  dark  and  could  be  read.  However,  no  use 
was  made  of  these  discoveries  for  the  purpose  of  making  pic- 
tures until  1802,  when  Wedgwood  published  a  paper  entitled 
"An  Account  of  a  Method  of  Copying  Paintings  on  Glass 
and  on  Making  Profiles  by  the  Agency  of  Light  upon  Nitrate 
of  Silver." 

This  reference  to  making  profiles  is  a  reference  to  one  of 
the  forms  of  portraiture  which  preceded  photography.  Be- 
fore portrait  photography  was  discovered,  there  were  people 
who  made  what  were  called  "silhouettes",  which  were  pro- 
file pictures  cut  out  of  black  paper  and  stuck  on  to  white 
paper.  Some  of  these  silhouettists  were  very  clever  indeed. 
Others  who  had  not  great  ability  arranged  their  sitter  so 
that  they  got  sharp  shadows  thrown  by  a  lamp  onto  a  white 
screen  and  this  gave  them  the  profile  to  copy.  Wedgwood 
thought  that  instead  of  cutting  out  the  silhouette  he  might 
print  this  profile  on  the  screen  by  using  paper  treated  with 
silver  nitrate,  which  would  darken  in  the  light.  Wedgwood 
not  only  used  his  new  process  to  record  these  silhouettes, 
but  he  tried  to  take  photographs  in  what  was  then  called 

7 


FUNDAMENTALS  OF  PHOTOGRAPHY 

the  "camera  obscura",  which  was  the  forerunner  of  the 
Kodak  of  to-day. 

The  camera  obscura  consisted  of  a  box  with  a  lens  at  one 
end  and  a  ground  glass  at  the  other,  just  like  a  modern 
camera.  It  was  used  by  artists  to  make  a  picture  of  anything 
they   wanted    to   draw,   as   by   observing    the   picture   on 


Fig.  1 
Silhouette  Picture  from  Old  Print. 

the  ground  glass  they  could  draw  it  more  easily.  Wedgwood 
tried  to  make  pictures  in  his  camera  obscura  by  putting  his 
prepared  paper  in  the  place  of  the  ground  glass.  His  paper 
however,  was  too  insensitive  to  obtain  any  result;  but  Sir 
Humphrey  Davy,  who  continued  Wedgwood's  experiments, 
using  chloride  of  silver  instead  of  nitrate,  succeeded  in  mak- 
ing photographs  through  a  microscope  by  using  sunlight. 
These  are  apparently  the  first  pictures  made  by  means  of  a 
lens  on  a  photographic  material. 

But  all  these  attempts  of  Wedgwood  and  Davy  failed  be- 
cause no  method  could  be  found  for  making  the  pictures 
permanent.  The  paper  treated  with  silver  chloride  or  silver 
nitrate  was  still  sensitive  to  light  after  part  of  it  had  dark- 
ened, and  if  it  were  kept  it  soon  went  dark  all  over  and  the 
picture  was  lost.  Davy  concluded  his  account  of  the  ex- 
periments by  saying:  "Nothing  but  a  method  of  prevent- 
ing the  unshaded  parts  from  being  colored  by  exposure  to 


THE  BEGINNINGS  OF  PHOTOGRAPHY 

the  day  is  wanting  to  render  this  process  as  useful  as  it  is 
elegant." 

This  much  needed  method,  however,  remained  wanting 
from  1802  until  1839,  when  Sir  John  Herschel  found  that 


Fig.  2. 
Crystals  of  Thiosulphate  of  Soda  or  "Hypo". 

"hypo",  which  he  had  himself  discovered  in  1819,  could  dis- 
solve away  the  unaltered  chloride  of  silver  and  enable  him  to 
"fix"  the  picture,  as  the  process  has  been  called  ever  since 
Herschel  made  the  discovery,  and  from  that  time  to  this 
hypo  has  been  the  mainstay  of  the  photographer,  enabling 
him  to  fix  his  pictures  after  he  has  obtained  them. 

The  early  processes  of  photography  required  very  great 
exposures  so  that  the  unfortunate  subject  had  to  sit  for  as 
long  as  ten  minutes  in  the  full  sun  without  moving  in  order 
to  impress  the  plate  sufficiently.  Although  many  experi- 
ments were  made  in  an  attempt  to  find  substances  more 
sensitive  to  light  so  that  the  exposure  could  be  reduced, 
the  only  real  solution  was  to  find  some  method  by  which 
light  had  to  do  only  a  little  of  the  work  and  the  produc- 
tion of  the  image  itself  could  be  effected  by  chemical  action 
instead  of  by  the  action  of  the  light. 

The  first  great  step  in  this  direction  was  taken  by  Fox 
Talbot  in  1841.  He  found  that  if  he  prepared  a  sheet  of 
paper  with  silver  iodide  and  exposed  it  in  the  camera  he  got 
only  a  very  faint  image,  but  if  after  exposure  he  washed  over 
the  paper  with  a  solution  containing  silver  nitrate  and  gallic 
acid,  a  solution  from  which  metallic  silver  is  very  easily  de- 
posited, then  this  solution  deposited  the  silver  where  the 


> 


FUNDAMENTALS  OF  PHOTOGRAPHY 

light  had  acted  and  built  up  the  faint  image  into  a  strong 
picture.  This  building  up  of  a  faint  image  or,  indeed,  of  an 
image  which  is  altogether  invisible,  into  a  picture  is  what  is 
now  called  "development".'  If  we  expose  a  film  in  the 
Kodak  and  then,  after  the  shutter  has  allowed  the  light  to 
act  for  a  fraction  of  a  second  on  the  film,  look  at  the  film  in 
red  light,  which  will  not  affect  it,  we  shall  not  be  able  to  see 
any  change  in  the  film.  But  if  we  put  the  film  into  a  devel- 
oping solution,  the  invisible  image  which  was  produced  by 
light,  and  which  in  photographic  books  is  called  "the  latent 
image"  will  be  developed  into  a  black  negative  representing 
the  scene  that  was  photographed. 

Fox  Talbot  was  not  only  the  first  to  develop  a  faint  or 
invisible  image;  he  was  also  the  first  man  to  make  a  nega- 
tive and  use  it  for  printing.  What  is  meant  by  a  negative 
is  this:  If  we  look  at  our  film  after  we  have  exposed  and 
developed  it,  we  shall  find  that  the  sky,  which  was  bright  in 
the  picture,  is  shown  in  our  film  as  very  black,  whileany  shad- 
ows in  the  picture,  which,  of  course,  were  dark,  will  be 

transparent  in  the  film,  so 
that  the  light  let  through 
the  film  is  in  the  reverse 
order  of  the  scene  photo- 
graphed, all  the  bright  parts 
in  the  scene  being  dark  in 
the  film  and  the  dark  parts 
bright.  For  this  reason  the 
film  is  called  a  "negative," 
and  when  it  is  printed  on 
paper  the  same  reversal 
happens  again  and  the  clear 
parts  in  the  negative  be- 
come dark  in  the  print 
while  the  dark  parts  of  the 
negative  protect  the  paper 
from  the  action  of  the  light, 
so  that  the  print  which  we  may  call  a  "positive,"  represents 
the  scene  as  it  appeared. 

Fox  Talbot,  then,  made  two  of  the  great  steps  in  the  ad- 
vancement of  photography  when  he  found  how  to  expose 
his  paper  for  a  time  insufficient  to  darken  it  completely,  and 
then  to  develop  a  negative  which  he  could  print  on  paper 
covered  by  silver  chloride.  Of  course,  the  paper  was  not 
transparent  as  our  film  is,  but  he  made  it  more  transparent 
by  treating  it  with  oil  or  wax.    In  this  he  was  followed  many 

10 


Fig.  3. 
Negative  Image. 


THE  BEGINNINGS  OF  PHOTOGRAPHY 


Positive  or  Print. 


years  afterwards  by  the  Eastman  roll  holder,  which  was  the 
forerunner  of  all  the  Kodaks.  In  this  roll  holder  at  first  a 
paper  film  was  used  to  make 
the  negative  and  then  the 
paper  was  made  transpar- 
ent for  printing. 

Fox  Talbot's  paper  nega- 
tives were  succeeded  by  the 
method  known  as  the  wet 
collodion  process,  which  has 
survived  to  the  present  day. 
This  is  the  process  chiefly 
used  by  photo-engravers  for 
making  the  negatives  from 
which  they  make  the  en- 
graved metal  plates  for 
printing  pictures. 

Collodion  is  made  by  dis- 
solving   nitrated    cotton, 
such  as  is  now  used  for  the  film  base,  in  a  mixture  of  ether 
and  alcohol.  The  worker  of  the  wet  collodion  process  had  to 

make  his  own  plates  at  the 
time  when  he  wanted  to 
take  a  picture.  He  would 
clean  a  piece  of  glass  and 
coat  it  with  the  collodion  in 
which  the  chemicals  were 
dissolved  and  then  put  the 
plate  in  a  bath  of  nitrate  of 
silver,  which  formed  silver 
iodide  in  the  collodion  film 
and  made  it  sensitive  to 
light.  Then  the  glass  had 
to  be  exposed  in  the  cam- 
era while  wet,  and  immedi- 
ately after  exposure  it  was 
developed  by  pouring  the 
developer  over  it.  It  was 
then  fixed  and  dried. 

In  order  to  carry  out 
these  operations  a  photo- 
grapher who  wanted  to 
take  landscapes  had  to  car- 
ry with  him  a  folding  tent 
which  he   could  set   up  in 

1 1 


Fig.  5. 

Early  Photographer  with  His  Equipment. 


FUNDAMENTALS  OF  PHOTOGRAPHY 

the  open  air.  The  tent  was  dark  except  for  a  yellow  or  red 
window  by  which  to  see  to  make  the  plates  and  develop 
them. 

All  this  difficulty  in  working  disappeared  with  the  coming 
of  the  gelatine  emulsion  process,  which  is  the  one  now  used. 
The  sensitive  coating  on  films  and  papers  now  consists  of  a 
bromide  or  chloride  of  silver  held  in  a  thin  sheet  of  gelatine 
the  gelatine  being  dissolved  in  hot  water,  the  silver  salt 
formed  in  the  solution,  and  the  warm  solution  of  gelatine 
containing  silver  then  coated  on  the  film  or  paper. 

The  gelatine  solution  with  the  silver  in  it  is  called  an 
"emulsion"  because  of  the  way  in  which  the  silver  remains 
suspended  in  the  gelatine.  The  first  gelatine  emulsions  were 
made  in  1871  by  Dr.  Maddox.  An  emulsion  made  in  much 
the  way  that  we  use  now  was  first  sold  in  1873  by  Burgess. 

At  first  the  early  experimenters  made  and  sold  the  emul- 
sion itself,  drying  it  for  sale  so  that  photographers  had  to 
take  this  dried  emulsion,  melt  it  up  in  hot  water,  and  coat 
it  on  their  plates.  After  a  time,  however,  people  realized 
that  this  was  a  great  deal  of  trouble  and  that  there  was  no 
reason  why  the  manufacturer  of  the  emulsion  should  not  coat 
the  glass  plates  with  it,  and  sell  the  ready  prepared  plates. 

In  those  days  all  negatives  were  made  on  glass  plates. 
These  plates  were  coated  with  the  emulsion  by  hand  and 
then  when  the  emulsion  was  spread  over  them  were  put 
on  to  cold  level  slabs  for  the  jelly  to  set  before  drying. 
Glass  plates  are  cumbersome  and  heavy,  and  for  this  reason 
George  Eastman  continually  experimented  to  substitute 
a  light,  flexible  support  for  the  brittle  and  heavy  glass.  As 
already  mentioned,  he  first  used  paper  as  a  support  for  the 
negative,  waxing  it  to  make  it  transparent  for  printing. 
This  was  followed  by  a  paper  from  which  the  film  carrying 
the  image  was  stripped,  the  film  being  transferred  to  a  glass 
plate  coated  with  gelatine  so  that  this  gelatine  made  a  sup- 
port for  the  film. 

While  experimenting  to  find  a  more  satisfactory  material 
for  coating  the  film  than  gelatine  it  was  found  that  a  solu- 
tion of  nitrated  cotton  would  make  a  clear,  transparent  and 
flexible  support,  and  after  a  period  of  further  experimenting 
this  material  was  adopted  and  a  roll  film  was  made,  the 
emulsion  being  carried  on  the  clear,  transparent  sheet  of 
film  support.  The  only  remaining  difficulty  with  this  was 
its  tendency  to  curl  owing  to  the  gelatine  coating  on  one 
side,  and  this  was  overcome  by  coating  the  other  side  with 
plain  gelatine,  thus  producing  the  non-curling  (NC)  film.^ 


CHAPTER  II. 

LIGHT  AND  VISION. 

Light  is  the  name  which  we  give  to  the  external  agency 
which  enables  us  to  see.  In  order  to  see  things  we  must  have 
something  which  enters  the  eye  and  a  brain  to  explain  it  to 
us.^  That  which  enters  the  eye  is  what  we  call  light. 

The  eye  consists  of  two  principal 
parts  and  can  best  be  understood 
by  analogy  with  the  camera.  In 
front  it  has  a  lens  which  forms  an 
image  on  the  sensitive  surface, 
which  is  called  the  retina,  the 
retina  playing  the  same  part  in  the 
eye  that  the  film  does  in  the  cam- 
era. The  retina,  however,  differs 
from  the  film  in  that  when  light 
falls  upon  the  film  it  produces  a 
permanent  change,  which   can   be  pjg  5 

developed    into   a   picture,    and    if         Diagram  of  Human  Eye. 
the  light  falls  upon  the  film  for  too 

long  a  time  the  film  is  spoiled,  while  the  retina  merely  acts 
as  a  medium  to  transmit  to  the  brain  the  sensation  of  the 
light  that  falls  upon  it,  and  when  the  light  stops,  the  sensa- 
tion stops  and  the  retina  is  ready  to  make  a  new  record.  The 
retina  behaves,  in  fact,  like  a  film  in  which  the  sensitive 
material  is  continually  renewed. 

It  is  probable  that  this  sensitive  material  in  the  eye  is 
really  of  a  chemical  nature  because  it  is  apparently  produced 
all  the  time,  and  when  the  eye  is  kept  in  the  dark  the  sensi- 
tive material  accumulates  for  some  time  so  that  the  eye  be- 
comes more  sensitive,  while  when  a  strong  light  falls  upon 
the  eye,  the  sensitive  substance  is  destroyed  more  rapidly 
than  it  is  produced  and  the  eye  becomes  less  sensitive. 

In  this  way,  the  eye  has  a  very  great  range  of  sensitive- 
ness. In  bright  sunlight  it  is  as  much  as  a  million  times  less 
sensitive  than  it  is  after  it  has  been  kept  for  an  hour  in  the 
dark,  and  it  changes  very  rapidly,  only  a  few  minutes  being 
necessary  for  an  eye  that  has  been  in  almost  complete  dark- 
ness to  adapt  itself  to  the  glare  of  out-door  lighting.  In 
order  to  lessen  the  shock  of  changing  light  intensity,  the 
lens  of  the  eye  is  provided  with  an  iris  diaphragm  just  like 


THE  FUNDAMENTALS  OF  PHOTOGRAPHY 


*"rrtl%* 


that  of  a  camera,  but  with  the  additional  advantage  that  it 
operates  automatically,  opening  and  closing  according  to 
the  intensity  of  the  light.    Measurements  of  the  movements 

of  the  iris  of  the  eye  have 
been  made  by  taking 
motion  pictures  of  the 
eye  when  suddenly  il- 
luminated by  a  bright 
light,  and  these  show 
what  a  wonderful  instru- 
ment the  eye  is  in  its 
adaptation  to  changing 
conditions  in  the  world 
around  it. 

The  retina  is  connect- 
ed with  the  brain  by  a 
great  many  nerve  fibers, 
each  fiber  coming  from  a 
different  part  of  the  ret- 
ina, so  that  when  light 
falls  upon  any  part  of 
the  retina,  the  intensity 
of  the  light  is  communi- 
cated by  the  tiny  nerve 
coming  from  that  part  of 
the  retina  to  the  brain 
and  the  brain  forms  an 
idea  of  the  image  on  the 
retina  by  means  of  the 
multitude  of  impressions 
from  different  parts  of 
the  retina. 

The  image  on  the  ret- 
ina is  inverted  like  all 
lens  images,  so  that  we 
really  see  things  stand- 
ing on  their  heads,  but 
the  brain   interprets   an 


Fis-  7- 

Iris  Opening  and  Closing. 


inverted  image  on  the  retina  as  corresponding  to  an  upright 
external  world,  and  although  the  eye  sees  things  upside 
down,  the  brain  has  no  idea  of  it. 

What  we  observe  is  the  light  which  falls  on  the  retina,  but 
this  light  comes  originally  from  some  external  source  which, 
in  the  case  of  daylight,  of  course,  is  the  sun.  The  light  from 
the  sun  is  reflected  by  the  objects  in  the  world  around  us 


LIGHT  AND  VISION 


according  to  their  nature,  and  entering  the  eye  it  enables 
us  to  see  the  objects.  When  we  look  at  a  landscape  we  see 
that  the  sky  is  bright  and  the  roads  and  fields  are  less  bright, 
and  the  shadows  under  the  trees  are  dark,  because  much  of 
the  light  of  the  sun  is  reflected  from  the  sky,  less  from  the 
fields  and  roads  and  still  less  from  the  shadows  under  the 
trees.  All  these  rays  from  the  sun  reflected  from  the  natural 
objects  in  the  landscape  enter  the  eye  and  make  a  picture  on 
the  retina  which  is  perceived  by  the  brain  by  means  of  the 
tiny  nerve  fibers  coming  from  the  retina  to  the  brain. 

But  the  eye  not  only  perceives  differences  in  the  bright- 
ness of  the  light — it  also  observes  differences  in  color — and 
in  order  to  understand  how  this  can  be  we  must  search 
further  into  the  nature  of  light  itself. 

The  nature  of  light  has  long  been  a  source  of  speculation, 
and  at  one  time  it  was  generally  held  that  the  light  which 
entered  the  eye  consisted  of  small  particles  shot  off  from  the 
source  of  light,  just  as  at  one  time  it  was  held  that  sound 
consisted  of  small  particles  shot  off  from  the  source  of  a 
sound  which  struck  the  drum  of  the  ear.  This  theory  of 
light  has  the  advantage  that  it  immediately  explains  reflec- 
tion; just  as  an  india  rubber 
ball  bounces  from  a  smooth  RED 

wall,  while  it  will  be  shot  in 
almost  any  direction  from  a 
heap  of  stones,  so  the  small 
particles  of  light  would  re- 
bound from  a  polished  sur- 
face at  a  regular  angle,  while 
a  rough  surface  would  mere- 
ly scatter  them. 

This  theory  of  the  nature 
of  light  was  satisfactory  un- 
til it  was  found  that  it  was 
possible  by  dividing  a  beam 
of  light  and  slightly  length- 
ening the  path  of  one  of  the 
halves,  and  then  reuniting 
the  two  halves  together 
again,  to  produce  alternate 
periods  of  darkness  and  light 
similar  to  the  nodes  of  rest 
produced  in  an  organ  pipe, 
where  the  interference  of  the  waves  of  sound  is  taking  place. 
It  could  not  be  imagined  that  a  reinforcement  of  one  stream 

l5 


GREEN 


BLUE 


Fig.  8. 

Relative  Wave  Lengths  of  Red,  Green 
and  Blue. 


FUNDAMENTALS  OF  PHOTOGRAPHY 

of  particles  by  another  stream  of  particles  in  the  same 
direction  could  produce  an  absence  of  particles,  while  the 
analogy  of  sound  suggested  that  just  as  sound  was  known  to 
consist  of  waves  in  the  air,  so  light  also  consisted  of  waves. 


BLUB  VI OUT 


One 

JAFM 


GREEN 


RBD 


Fig.  9. 
Simple  Arrangement  of  Spectrum. 

Light  cannot  consist  of  waves  in  the  air,  partly  because 
we  know  that  it  travels  through  interstellar  space,  where 
we  imagine  that  there  is  no  air  but  through  which  we  can 
still  see  the  light  of  the  stars,  and  also  because  the  ve- 
locity of  light — nearly  200,000  miles  per  second — is  so  great 
that  it  is  impossible  that  it  could  consist  of  a  wave  in  any 
material  substance  with  which  we  are  acquainted.  It  is, 
therefore,  assumed  that  there  exists,  spread  through  all 
space  and  all  matter,  something  in  which  the  waves  of  light 
are  formed,  and  this  something  is  termed  ether,  so  that  it  is 
generally  held  that  light  consists  of  waves  in  the  ether. 

Just  as  in  sound  we  have  wave  notes  of  high  frequency, 
that  is,  with  many  waves  per  second  falling  upon  the  ear, 
which  form  the  high  pitched  notes,  and  also  notes  of  low 
frequency  where  only  a  few  waves  a  second  fall  upon  the 
ear  forming  the  bass  notes,  so  with  light  we  may  have  dif- 
ferent frequencies  of  vibration.  Since  the  velocity  of  light 
is  the  same  for  waves  of  different  frequencies,  it  is  clear  that 
the  waves  of  high  frequency  will  be  of  different  wave  length 
from  those  of  low  frequency,  the  wave  length  being  the  dis- 
tance from  the  crest  of  one  wave  to  the  crest  of  the  next, 
and  if  we  obtain  waves  of  different  lengths  separated  out, 
we  shall  find  that  the  color  depends  upon  the  wave  length. 
Fig.  8  shows  the  average  length  of  wave  corresponding  to 
light  of  various  colors,  the  diagram  being  drawn  to  scale. 

White  light  consists  of  mixtures  of  waves  of  various 
lengths,  but  if  instead  of  letting  the  mixture  of  waves,  which 
forms  white  light,  fall  directly  on  the  eye  we  pass  white 
light  through  an  instrument  known  as  a  spectroscope, 
which  changes  the  direction  of  the  different  waves  by 
amounts  which  differ  according  to  their  lengths,  we  get  the 
white  light  spread  out  into  a  band  of  colors  which  we  call 

16 


LIGHT  AND  VISION 

the  spectrum,  and  we  can  scale  this  spectrum  by  means  of 
numbers  representing  the  lengths  of  the  waves. 

Fig.  9  gives  a  simple  arrangement  of  the  spectrum,  the 
numbers  representing  the  wave  lengths  in  units  which  are 
ten-millionths  of  millimeters.  It  will  be  seen  that  the  visi- 
ble spectrum  extends  from  7,000  to  4,000  units,  wave  lengths 
of  7,000  units  corresponding  to  the  extreme  red  and  4,000 
to  the  darkest  violet  that  can  be  seen,  while  the  brightest 
region  of  the  spectrum  stretches  from  5,000  to  6,000  units 
and  includes  the  green  and  yellow  colors.  The  spectrum 
is  equally  divided  into  three  regions  which  may  be  broadly 
termed— red  7,000-6,000,  green  6,000-5,000,  and  blue- 
violet  5,000-4.000. 

If  we  get  a  piece  of  colored  glass  which  lets  through  only 
the  portion  of  the  spectrum  between  6,000  and  7,000,  then 
we  should  have  a  piece  of  red  glass,  a  glass  which  let  through 
from  5,000  to  6,000  would  be  a  green  glass,  and  one  which 
let  through  from  4,000  to  5,000  would  be  blue-violet  in  color, 
so  that  from  the  spectrum  we  already  derive  the  idea  that 
light  can  be  conveniently  divided  into  three  colors,  which 
we  may  call  the  primary  colors — red,  green  and  blue-violet. 
It  is  probable  that  this  is  connected  with  the  structure  of 
the  retina,  and  one  theory  holds  that  there  are  three  sets  of 


Fig.  10. 
Portions  of  Spectrum  Transmitted  by  Primaries. 

17 


FUNDAMENTALS  OF  PHOTOGRAPHY 

receiving  nerves  in  all  parts  of  the  retina,  corresponding  to 
the  three  primary  colors — red,  green  and  blue-violet. 

If  we  let  white  light  fall  upon  anything,  such  as  a  piece  of 
white  paper,  which  reflects  all  the  wave  lengths  to  the  same 
extent,  then  the  reflected  light  remains  white  and  we  should 
say  that  the  object  on  which  it  falls  is  uncolored,  but  if  the 
object  absorbs  some  of  the  wave  lengths  of  the  spectrum 
more  than  others,  then  it  will  appear  colored.  Thus,  a 
piece  of  red  paper  appears  red  because  from  the  white  light 
falling  upon  it  it  absorbs  some  of  the  green  and  blue-violet 
light,  but  reflects  all  the  red  light  and,  therefore,  appears 
red.  In  the  same  way  a  green  object  absorbs  both  red  and 
blue-violet  more  than  it  absorbs  the  green  light  and  so  looks 
green,  and  a  yellow  object  absorbs  the  blue,  reflecting  the 
red  and  green  of  the  spectrum  and  so  appears  yellow. 

Light  waves  differ  not  only  in  their  length  but  in  their 
amplitude,  that  is,  in  the  height  of  the  wave,  and  the  ampli- 
tude controls  the  intensity  of  the  light  just  as  the  wave 
length  controls  the  color.  The  eye,  therefore,  can  detect 
differences  in  brightness  which  depend  upon  amplitude,  and 
also  differences  of  color  which  depend  upon  wave  length. 


i8 


CHAPTER  III. 


ABOUT  LENSES. 

IN  order  to  take  a  photograph  we  use  a  lens  which  forms 
an  image  of  the  object  we  want  to  photograph  upon  the 
film.  The  simplest  lens  which  we  could  use  would  be  a 
small  hole.  Suppose  that  we  take  a  sheet  of  cardboard  and 
make  a  hole  in  it  with  a  pin,  and  then,  in  a  darkened  room, 
hold  the  cardboard  between  a  sheet  of  white  paper  and  an 
electric  lamp;  we  shall  see  on  the  paper  an  image  of  the  lamp 
filament. 

The  diagram  shows  how 
this  image  is  produced.  A 
ray  of  light  from  each  por- 
tion of  the  filament  passes 
through  the  pinhole  and  forms 
a  spot  of  light  on  the  paper, 
and  all  these  spots  joining  to- 
gether form  the  image  of  the 
filament. 

If  we  take  the  lens  out  of  a 
camera  and  replace  it  by  a 
thin   piece  of  metal    pierced 

with  a  hole  made  by  a  needle  (a  No.  10  sewing  needle  is 
about  right,  and  the  edges  of  the  hole  must  be  beveled  off 
so  that  they  are  sharp),  then  we  can  take  excellent  photo- 
graphs by  giving  sufficient  exposure. 

If  the  pinhole  is  about  six  inches  from  the  film  then  an 
exposure  of  about  one  minute  for  an  outdoor  picture  on 
film  will  be  required.  It  is  necessary,  of  course,  to  make  a 
well  fitting  cap  for  the  lens  aperture  so  that  no  light  will 
get  in  except  through  the  pinhole,  and  also  to  make  a 
cover  for  the  pinhole  to  act  as  a  shutter  for  exposing. 

But  if  a  pinhole  were  the  only  means  of  forming  an  image 
it  is  very  improbable  that  photography  would  ever  have 
been  developed,  since  the  exposures  are  so  long  in  conse- 
quence of  the  small  amount  of  light  which  can  pass  through 
the  pinhole. 

!9 


Fig.  11. 
How  an  Image  is  Produced. 


FUNDAMENTALS  OF  PHOTOGRAPHY 


Fig.  12. 
Pinhole  Image  of  a  Star. 


In  order  to  get  more  light  we  could  try  making  the  pin- 
hole larger,  but  the  effect  of  this  is  to  make  the  image  very 
indistinct,  and  even  the  smallest  efficient  pinhole  can  not 

give  as  sharp  an  image  as  a 
good   lens. 

Suppose  we  have  a  small 
pinhole  forming  an  image  of 
a  star,  as  shown  in  Fig.  12. 

If  we  make  the  hole  larger, 
we  shall  get  a  round,  spread- 
ing beam  of  light  and  no  long- 
er get  a  sharp  image.  (Fig. 
13.) 

What  we  need,  if  we  are  to 
use  the  large  hole  is,  some 
means  of  bending  the  light  so  that  all  the  light  reaching 
the  hole  from  the  star  is  joined  again  in  a  sharp  image  of 
the  star  on  the  screen,  as 
shown  in  Fig.  14. 

If  a  ray  of  light  falls  on  a 
piece  of  glass  so  that  it  is  not 
perpendicular  to  it,  it  will  be 
bent.  There  is  an  interesting 
experiment  which  shows  this 
very  well.  Take  a  thick  block 
of  glass  and  place  it  so  that  it 
touches  a  pin  (which  is  mark- 
ed B  in  Fig.  15)  and  stick  an- 
other pin  (A)  in  the  board. 
Now  look  through  the  glass 
and  stick  a  pin  (D)  between  your  eye  and  the  glass,  and  in 
the  same  line  of  sight  as  A  and  B,  and  lastly  another  pin 

(C)  touching  the  glass  and 
in  the  same  line  of  sight  as 
the  other  three. 

Take  away  the  glass  and 
join  up  the  pinholes  with  pen- 
cil lines.  You  will  find  that 
the  line  DC  is  parallel  to  the 
line  AB  but  is  not  in  the  same 
line;  that  is,  the  ray  of  light 
marked  by  the  litre  AB  was 
iv-    j    f  ^i      g"      n      i  i  •  i.       bent  when  it  entered  the  class 

Need  of  Means  to  Bend  Light.  ,     ,,  ,         ,     ,        ,        *     . 

and    then    bent    back    again 
when  it  left  it,  so  we  can  bend  light  by  means  of  glass. 

20 


Fig.  13. 
Effect  of  Large  Pinhole. 


ABOUT  LENSES 


If  we  take  a  triangular  piece  of  glass  (called  a  prism)  we 
can  bend  a  ray  when  it  enters  the  glass  and  also  more  still 
when  it  leaves  the  glass.    (Fig.  17.) 

And  a  lens  is  really  two 
prisms  stuck  together  base 
to  base  (Fig.  18).  So  that 
if  we  put  a  lens  in  the  hole 
with  which  we  want  to 
form  an  image,  we  can  do 
what  we  wish  to  and  make 
all  the  rays  from  the  star 
come  together  again  in  the 
image  of  the  star.  And  this 
is  the  purpose  of  our  cam- 
era lenses,  to  form  an  image 
as  sharp  as  that  given  by 
the  smallest  pinhole  and  yet 


^B/^ 

v\ 

/D. 

Fig.  15. 
Deflecting  a  Ray  of  Light. 


much  brighter  than  any  pin  hole  would  give. 

Should  we  place  a  pinhole,  instead  of  a  lens,  in  the  front- 
board  of  our  camera,  we  could  use  the  same  size  of  pinhole 
for  making  all  sizes  of  pict- 
ures, because  the  image 
formed  by  a  pinhole  is  al- 
ways of  the  same  sharpness, 
whether  the  pinhole  is  far 
from  the  film  or  close  to  it. 
If  we  want  a  large  picture 
we  must,  of  course,  use  a 
large  camera  with  a  long 
bellows,  so  the  pinhole  will 
be  a  long  way  from  the 
film,  while  if  we  want  a 
small  picture  we  shall  only 
need  a  small  camera  with 
a  short  bellows,  so  the  pin- 
hole will  be  near  the  film.  But  if,  instead  of  a  pinhole,  we 
use  a  lens,  we  shall  find  that  the  lens  must  be  placed  at  a 

certain  distance  from  the  film 
(depending  upon  its  focal 
length  and  its  distance  from 
the  object  photographed)  in 
order  to  obtain  a  sharp  pic- 
ture. If  it  is  placed  at  any 
other  distance  from  the  film 


Fig.  16. 
Path  of  Deflected  Ray. 


PRIJAA 

Fig.  17. 
Prism  Bending  a  Ray, 


the  picture  will  be  all  blurred.    The  reason  for  this  is  that 


21 


FUNDAMENTALS  OF  PHOTOGRAPHY 


Fig.  IS. 
Rays  Bent  by  Double  Prism. 


a  photographic  lens  bends  the  rays  of  light  that  pass  through 
it  so  that  all  the  light  rays  from  a  star,  for  instance,  will 
meet  again  to  form  an  image  of  the  star.  By  placing  a  sheet 
of  cardboard  at  the  position  where  the  rays  of  light  meet, 
the  image  of  the  star  will  be  sharp,  but  if  we  put  the  card 

either  nearer  to  or  farther 
from  the  lens,  the  image  will 
be  blurred  into  a  circle  of 
light.  The  distance  at  which 
the  lens  must  be  placed  from 
the  film  to  give  a  sharp  image 
represents  the  "focal  length" 
of  the  lens. 

The  longer  the  focal  length  of  a  lens  the  larger  the  image, 
and  the  shorter  the  focal  length  the  smaller  the  image. 
Suppose  we  photograph  a  tree 
and  place  the  camera  at  such 
a  distance  from  the  tree  that 
with  a  lens  of  three  inches 
focal  length  we  obtain  a  pic- 
ture in  which  the  image  of 
the  tree  is  one  inch  long. 

Now,  if  with  the  camera  at 
the  same  distance  from  the 
tree,  we  had  used  a  six-inch 
lens  instead  of  the  three-inch 
lens,  which  means  that  in- 
stead of  the  lens  being  three 
inches  from  the  film  it  would 


Fig.  19. 
I. ens  Forming  a  Sharp  Image. 


be  six  inches  from  it,  then  the  image  of  the  tree  would  be 
two  inches  long  instead  of  one  inch  long  in  the  picture.    If 

.->    we  were  using  the  same  size 
-|    film     with     both     lenses,     of 
!    course  we  should  not  be  able 
to  include  as  much  of  the  sub- 
ject  we  were  photographing 
i    in  the  field  of  view  of  the  pic- 
~~J    ture  made  with   the  six-inch 
Fig.  20  iens  as  we  should  obtain  with 

I  mages  Formed  by  a  pinhole  at  various 

distances.  _.,_., 

the  three-inch  lens,  because 
with  the  three-inch  lens  the 
tree  would  be,  say,  a  quarter 
of  the  length  of  the  film, 
w  hili-  with  the  six-inch  lens  it 


Fig.  21. 

A  lens  forms  an  image  at  only  one  point. 


22 


ABOUT  LENSES 

would  be  half  the  length  of  the  film.  In  other  words,  the 
three-inch  lens  would  give  us  a  smaller  image,  while  the 
six-inch  lens  would  give  us  a  large  image  of  the  tree. 


fOC At-  LENGTH 


Fig.  22. 
Short  Focal  Length  Means  Small  Image. 


The  longer  the  focal  length  of  a  lens,  the  less  subject  we 
include  in  our  picture,  and  the  larger  the  images  of  objects 
are,  while  the  shorter  the  focal  length,  the  more  subject 
we  include  in  the  picture  and  the  smaller  the  images  are. 

In  actual  practice  we  must  compromise  between  a  lens 
which  will  include  as  large  an  area  as  possible  in  the  field 
of  view,  and  a  lens  which  will  give  images  as  large  as  pos- 
sible; consequently,  for  general  all-around  purposes  it  is  best 
to  use  a  lens  whose  focal  length  is  somewhat  longer  than  the 
longest  side  of  the  film.  For  a  23^  x  4^  film,  for  instance, 
we  should  use  a  lens  of  about  5  inches  focal  length. 

It  is  most  important  not  to  use  a  lens  of  too  short  a  focal 
length  for  the  size  of  the  film  employed.  There  is  a  great 
temptation  to  do  this.  While  a  lens  of  \x/i  inch  focus  as  com- 
pared with  a  lens  of  three  inch  focus  means  a  big  lens  in  place 


FOCAL   LENGTH 

Fig.  23. 
Long  Focal  Length,  Larger  Image. 

of  a  little  lens,  and  a  larger  shutter  and  a  somewhat  larger 
camera  in  place  of  a  smaller  shutter  and  an  extremely  com- 
pact camera,  it  also  means  (and  this  is  vastly  more  impor- 
tant than  mere  camera  compactness)  the  making  of  pictures 
having  good  perspective  instead  of  pictures  with  bad  per- 

23 


FUNDAMENTALS  OF  PHOTOGRAPHY 


spective;  in  other  words,  it  means  pictures  the  drawing  in 
which  looks  right  instead  of  pictures  whose  drawing  looks 

wrong.  The  reason  for 
this  is  that  the  per- 
spective of  a  picture 
is  determined  by  the 
point  of  view  from 
which  the  lens  makes 
the  picture.  If  this 
perspective  is  not 
pleasing  to  the  eye  it 
will  not  be  pleasing 
in  the  picture. 

Fig.  24  shows  a  pic- 
ture made  with  a  very 
short  focus  lens  used 
close  to  the  subject. 
This  is  a  faithful  ren- 
dering of  the  perspec- 
tive that  the  eye  saw 
from  the  viewpoint  of 
the  lens,  and  is  far 
from  pleasing. 

In  Fig.  25  the  same 
subject  is  shown 
photographed  with  a 
long  focus  lens,  and 
in  this  picture  the 
perspective  is  satis- 
factory. It  likewise 
represents  the  per- 
spective that  the  eye 
saw  from  the  view- 
point of  the  lens. 

It  is  a  good  rule  to 
secure  a  lens  which 
has  a  focal  length  at 
least  equal  to  the 
diagonal  of  the  film. 
A  little  more  focal 
length  is  still  better. 
Lenses  differ  in  an- 
other respect  than 
their  focal  length. 
Fig.  25.    Made  With  a  Long  Focus  Lens.      They    differ    in    the 

24 


Fig.  24. 
Made  With  a  Very  Short  Focus  Lens. 


ABOUT  LENSES 


^ 

,- 

r"X' ! 

\      >      I  .--!--*"> 

II        '        1        1 

— r~  "i 

1 

L~— 

Fig.  26. 
Visible  Area  with  Long  Focus. 


amount  of  light  they  admit,  and  this  is  very  important, 
because  the  more  light  admitted,  the  shorter  the  exposure 

can  be.  The  chief  object  in 
using  a  lens  instead  of  a  pin- 
hole is  to  transmit  more  light 
to  the  film,  and  the  amount 
of  light  that  is  transmitted 
depends  upon  the  area  of  the 
glass  in  the  lens. 

Suppose  we  place  a  piece 
of  cardboard,  instead  of  a 
film,  in  the  back  of  a  camera, 
and  have  a  pinhole  in  the 
card  through  which  we  can 
look  at  the  lens;  then  point  the  lens  toward  a  window;  the 
amount  of  light  that  reaches  the  eye  through  the  hole  in 
the  card  depends  upon  how  much  of  the  light  from  the 
window  is  passing  through  the  lens;  that  is  to  say,  it  will 
depend  on  the  area  of  the  window  which  we  could  see  if 
there  was  no  glass  in  the  lens.  Of  course,  since  the  visible 
area  of  the  window  is  bounded  by  the  edges  of  the  lens 
mount,  we  could  see  more 
if  the  lens  were  of  shorter 
focal  length  so  that  the  eye 
was  closer  to  it.  With  a 
lens  of  long  focal  length 
only  a  small  part  of  the 
window  area  is  visible. 

With  a  lens  of  half  the 
focal  length  but  of  the 
same  diameter  as  that 
shown  in  Fig.  26,  four  times 
as  much  of  the  window  area 
is  visible. 

The  brightness  of  the  image  projected  by  lenses  of  the 
same  diameter  varies  inversely  as  the  square  of  the  focal 
length  of  the  lens.  It  also  varies  as  the  area  of  the  lens 
surface  (aperture)  which  admits  the  light.  The  greater 
the  lens  aperture  the  more  light  it  admits.  Now  the  area  of 
the  lens  aperture,  of  course,  is  proportional  to  the  square 
of  its  diameter,  so  that  all  lenses  in  which  the  diameter 
of  the  aperture  bears  the  same  ratio  to  the  focal  length 
will  give  equally  bright  images.  This  means  that  the  bright- 
ness of  the  image  is  determined  not  solely  by  the  focal 
length,  nor  solely  by  the  diameter  of  the  lens  aperture,  but 

25 


r*~]    L^r--r~r  ! 

r "" "" 1 


Fig.  27.  ""-L 

Visible  Area  with  Short  Focus. 


' 


FUNDAMENTALS  OF  PHOTOGRAPHY 


by  the  relation  that  exists  between  the  lens  aperture  and 
the  focal  length  of  the  lens,  so  that  all  lenses  in  which  the 
diameter  of  the  opening  is,  say,  one-sixth  of  the  focal  length, 
will  give  equally  bright  images.  Thus,  in  a  lens  of  one-inch 
aperture  and  a  focal  length  of  six  inches,  the  opening  is 
one-sixth  of  the  focal  length,  and  in  a  lens  of  twelve  inches 
focal  length  and  two  inches  aperture,  the  opening  is  like- 
wise one-sixth  of  the  focal  length.  Both  lenses  are  of  the 
same  /  value.  This  means  that  both  give  an  image  of  the 
same  brightness,  and  will  require  the  same  exposure.  Lens 
"apertures"  are,  therefore,  rated  according  to  the  ratio  be- 
^  tween  their  diameter  and  their  focal  lengths;  thus,  one  in 
which  the  opening  is  one-sixth  of  the  focal  length  is  marked 
/.6;Jone  in  which  the  opening  is  one-eighth, /.8,  and  so  on, 
and  the  larger  the  aperture,  the  more  light  the  lens  trans- 
mits, and  the  more  light  it  transmits  the  shorter  the  expos- 
ure needed. 

But  while  large  lens  apertures  have  the  advantage  of 
permitting  shorter  exposures,  they  have  some  disadvant- 
ages. In  the  first  place,  to  get  a  large  aperture  we  must 
have  a  large  lens,  and  this  means  an  expensive  lens;  also, 
the  errors  of  definition,  which  are  called  the  "aberrations" 
of  lenses,  increase  very  much  as  the  apertures  increase,  so 
that  only  the  very  best  types  of  lenses  in  which  these  aberra- 
tions are  removed  to  as  great  an  extent  as  possible,  can  be 
made  of  large  aperture  and  still  give  good  definition.  Large 
aperture  lenses  are  therefore  costly. 

But  even  when  we  have  a  lens  with  a  large  aperture  we 
shall  have  to  regard  this  as  a  reserve  power  for  use  in 
special  circumstances,  and  we  shall  not  by  any  means  be 
able  to  use  it  at  its  largest  aperture  all  the  time. 

From  the  construction  of  a  lens  it  follows  that  only  the 
rays  from  a  mathematical  point  can  come  together  in  a 
point  again,  and  that  the  rays  from  any  point  nearer  or 
farther  than  the  point  focused  can  not  meet  in  a  point 
image  on  the  film,  but  must  produce  a  small  disc  of  light 
instead  of  a  sharp  point  of  light.    (Sec  Fig.  21.) 

The  disc  is  termed  the  circle  of  confusion.  If  the  circle 
of  confusion  is  small  enough  we  shall  not  be  able  to  dis- 
tinguish it  from  a  point,  and  the  picture  will  appear  to  be 
sharp. 

With  what  are  known  as  "fixed  focus"  cameras,  such  as 
the  Vest  Pocket  Kodaks  and  the  Box  Brownies,  no  attempt 
is  made  to  secure  a  wholly  sharp  focus  for  objects  at  all 
distances,  but  the  cameras  are  sharply  focused  on  the  near- 

26 


ABOUT  LENSES 


est  point  to  the  camera  which  will  still  enable  distant  ob- 
jects to  appear  approximately  sharp  in  the  pictures,  and  in 
this  way  objects  in  the  middle  distance  are  perfectly  sharp, 
and  near  objects  are  also  sharp,  provided  they  are  not  too 
near. 

The  following  table  of  these  distances,  beyond  which 
everything  is  sharp  when  the  largest  stop  is  used,  may  be 
useful: 

Vest  Pocket  Kodak    ....       9      feet 

No.  0  Brownie 9 

No.  2  Brownie 13^    " 

No.  2A,  2C  and  No.  3  Brownie     .     15 

If  we  are  using  a  No.  0  Brownie,  for  instance,  as  long  as 
everything  is  farther  off  than  nine  (9)  feet  we  can  rely  on 
getting  a  picture  with  everything  focused  sharply. 

With  the  focusing  Kodaks  we  must  judge  the  distance 
of  the  object  on  which  we  wish  the  focus  to  be  sharpest 
and  set  the  scale  to  that;  then  we  shall  find  that  objects 
somewhat  nearer,  and  also  objects  a  good  deal  farther  from 

the  camera  are  also  sharp, 
and  the  distance  from 
the  nearest  to  the  far- 
thest objects  that  ap- 
pear sharp  in  the  nega- 
tive is  called  the  "depth 
of  focus. "This  depth 
locus depends  on  the 
focal  length  of  the  lens  and  on  the  size  of  stop  used  in  the 
lens;  the  greater  the  focal  length  the  less  the  depth  of  focus, 
and  the  bigger  the  stop  the  less  the  depth  of  focus.  Thus  in 
Fig.  28,  we  have  a  lens 
focusing  near  and  far 
points  at  full  aperture 
and  producing  large  cir- 
cles of  confusion.  In  Fig. 
29  a  smaller  stop  is  used 
in  the  same  lens,  and  the 
circles  diminish  in  size  in  proportion  to  reduction  in  the 
size  of  the  stop. 

Sometimes  we  have  to  focus  near  objects  at  the  same 
time  as  distant  ones,  so  that  it  is  necessary  to  "stop  the 
lens  down"  to  some  extent. 

Stops  are  marked  on  two  different  systems,  though  both 
are  based  on  the  fundamental  ratio  of  the  diameter  to  the 

27 


Fig.  28. 

Depth  of  Focus  with  Full  Aperture. 


Fig.  29. 
Depth  of  Focus  with  Smaller  Aperture. 


FUNDAMENTALS  OF  PHOTOGRAPHY 

focal  length  of  the  lens.  In  the  one  system  the  stop  is  ex- 
pressed simply  as  a  fraction  of  the  focal  length;  thus  F./8 
(commonly  written  /.8)  means  that  the  aperture  is  one- 
eighth  of  the  focal  length  of  the  lens;  /.16,  one-sixteenth, 
and  so  on.  The  rectilinear  lenses  fitted  to  Kodaks  are,  how- 
ever, marked  in  the  "Uniform  System"  (U.  S.)  in  which 
the  numbers  are  proportional  to  the  exposure  required,  fA 

being  taken  as  unity,  so  that  the  scale  is  as  follows: 

! 

F.    fA    /.5:6    f.6.3    f.S    /.ll    /.16    f.22    f.32    /.45 
U.  S.      1        2  23^      4       8  16       32       64     128 

The  U.  S.  numbers  give  the  relative  exposure  that  is  re- 
quired with  the  /.  system  stops,  the  exposure  varying  as 
the  square  of  the/,  value,  so  that/.  11  requires  twice  the 
exposure  of/.8;/.16  twice  that  of/.  11  and  so  on. 

Kodaks,  Premo  and  Brownie  cameras  are  listed  with 
several  different  kinds  of  lenses,  the  smaller  cameras  being 
listed  with  either  Meniscus,  Meniscus  Achromatic,  Rapid 
Rectilinear  or  Anastigmat  Lenses.  The  larger  cameras  have 
either  Rapid  Rectilinear  or  Anastigmat  Lenses,  while  the 
Special  Kodaks  and  Graflex  cameras  have  Anastigmats  only. 
The  Box  Brownies  are  equipped  with  Meniscus  or  Meniscus 
Achromatic  Lenses,  while  with  the  Folding  Brownies  there 
is  a  choice  between  Meniscus  Achromatic  and  Rapid  Recti- 
linear lenses. 

Many  people  do  not  understand  the  meaning  of  these 
terms,  and  while  it  is  a  safe  rule  to  choose  the  best  lens 
which  can  be  afforded,  certain  that  the  better  lens  is  worth 
the  extra  cost,  it  is  still  better  to  understand  the  properties 
of  the  different  kinds  of  lenses  and  what  advantages  can  be 
gained  from  the  use  of  the  higher  grades. 

The  simplest  lenses  which  can  be  used  are  made  of  a  single 
piece  of  glass,  the  form  of  the  lens  being  of  the  type  which 
gives  the  best  definition;  that  is,  a  Meniscus  or  crescent 
shape,  and  the  lenses  are  called  Meniscus  (not  Meniscus 
Achromatic)  lenses.  Such  a  Meniscus  lens  can  only  be 
used  in  a  fixed  focus  camera  where  the  maker  of  the  camera 
has  put  it  in  the  correct  position  for  forming  a  sharp  image 
upon  the  film,  if  such  a  lens  were  used  in  a  focusing 
camera  we  should  find  that  however  carefully  we  focused 
the  picture  on  the  ground  glass  the  negatives  would  not  be 
sharp,  unless  the  difference  between  the  focusing  point 
of  the  visual  rays  by  which  we  focus,  and  the  chemical  rays 
which  affect  the  film,  was  provided  for. 

28 


ABOUT  LENSES 

This  is  because  a  non-achromatic  lens  bends  the  rays  of 
light  of  different  colors  to  different  extents,  so  that  the 
yellow  rays  which  we  use  for  focusing  do  not  come  to  a 
focus  in  the  same  place  as  the  blue  rays  which  affect  the 
film,  because  the  blue  rays  are  bent  more  than  the  yellow. 

In  1752,  Dollond,  an 
English  optician,  showed 
that  by  combining  two  dif- 
ferent kinds  of  glass  to 
make  a  lens  he  could  get 

Fig-  30.  the  blue  rays  to  focus  at 

Focus  of  Blue  and  Yellow  Rays.        the  same  point  ag  the  yd_ 

low  rays,  and  lenses  made  in  this  way  were  called  "achro- 
matics,"  from  the  Greek  words  "a"  meaning  not,  and 
"chroma"  meaning  color.  The  best  shape  of  achromatic 
lens  to  use  is  shown  in  Fig.  31,  and  since  this  is  also  of  a 
"meniscus"  or  crescent  shape  the  lenses  are  called  meniscus 
achromatics.  If  a  single  achromatic  lens  is  used,  it  is  neces- 
sary to  "stop  it  down"  so  that  only  a  small 
portion  of  the  lens  is  used,  because  the  rays 
which  come  through  the  edges  do  not  focus 
together  as  well  as  those  which  come  through 
the  center,  and  so  the  image  is  not  quite  sharp 
if  the  whole  lens  area  is  used. 

This  stopped-down  meniscus  lens  has  the 
effect  of  producing  slight  curvature  of  the  Fig.  31. 
edges  of  the  picture,  which  does  not  matter  Achromatlc  Lens 
in  landscape  work  or  portraiture;  but  if  subjects  containing 
straight  marginal  lines  are  photographed  with  such  a  lens, 
their  outer  lines  appear  slightly  curved — so  slightly,  how- 
ever, that  the  effect  is  negligible  unless  the  image  of  the 
subject  so  crowds  the  picture  area  that  its  outer  lines  are 
very  near  the  margins  of  the  picture,  as  shown  by  figures 
32  and  33,  which  represent  a  window  sash  photographed 
with  a  meniscus  lens  at  short  range. 

If  the  stop  is  in  front  of  the  lens  the  curvature  is  in  one 
direction,  and  if  it  is  behind  the  lens  the  curvature  is  in  the 
opposite  direction,  so  that  if  we  put  two  achromatics  to- 
gether with  the  stop  between  them,  the  curvature  is  neutra- 
lized and  we  get  a  lens  which  gives  no  curvature  at  all. 

Such  a  lens  is  called  a  "Rapid  Rectilinear" —  rectilinear 
because  it  gives  straight-line  images,  and  rapid  because 
having  a  focal  length  half  that  of  either  of  the  component 
achromatics  with  a  stop  of  the  same  diameter,  it  passes 
four  times  as  much  light  and  only  requires  one-quarter  of 

29 


FUNDAMENTALS  OF  PHOTOGRAPHY 

the  exposure.  Rapid  Rectilinears  are  sometimes  called  by 
other  names,  such  as  "Rapid  Aplanats,"  "Planatographs," 
and  so  on.  Now,  it  so  happens  that  the  two  kinds  of  glass 
used  in  an  achromat  must  fulfill  certain  conditions  to  bring 
the  blue  and  the  yellow  rays  to  the  same  focus,  and  must 


1 

1 

- 

- 


Fig.  32. 

Made  with  Stop'in  Front 
of  MeniscusJLens 


Fig.  33. 

Made  with  Stop  Behind 
Meniscus  Lens. 


Fig.  34. 

Made  with  Rectilinear 
Lens 


fulfill  certain  other  conditions  to  get  a  picture  which  is  flat, 
that  is,  a  picture  that  is  sharp  on  a  flat  plate  or  film;  and 
the  ordinary  glasses  which  are  used  for  making  achromats 
will  not  fulfill  all  these  conditions  at  once,  so  that  the  lenses 
made  with  "old"  achromats  will  not  give  flat  field  images, 
the  image  being  saucer-shaped.  These  lenses  are,  therefore, 
said  to  be  "astigmatic,"  which  means  that  they  do  not  give 
sharp-point  images  of  points. 

About  thirty  years  ago,  Professor  Abbe  and  Otto  Schott, 
working  together  at  Jena,  found  out  how  to  make  new  kinds 
of  optical  glass  from  which  lenses  could  be  made  which 
would  give  flat  field  images  with  the  blue  and  yellow  rays 
of  the  same  focus. 

By  the  use  of  these  new  glasses  the  opticians  have  been 
able  to  make  lenses  that  give  sharp  images  on  a  flat  field  to 
the  very  edge  of  the  picture  and,  therefore,  these  lenses  are 
called  "Anastigmats,"  meaning  "not  astigmatic,"  but  this 
better  defining  power  can,  however,  only  be  obtained  by 
the  most  careful  and  skilled  work  in  making  the  lens,  this 
work  being  of  a  far  higher  quality  than  that  employed  on 
the  older  types  of  lenses,  which  accounts  for  the  higher  cost 
of  anastigmats. 

Anastigmat  lenses  can  be  used  with  larger  stops  than 
any  of  the  older  lenses,  so  that  if  an  Achromatic  working 

3° 


ABOUT  LENSES 

at/.  16  requires  a  1/5  second  exposure,  a  Rapid  Rectilinear 
working  at/.8  will  require  a  1/20  second  exposure,  and  an 
Anastigmat  working  at  f.6.3  will  require  a  1/32  second  ex- 
posure. 

To  summarize  the  advantages  and  disadvantages  of  the 
three  types  of  lenses  discussed  in  the  preceding  pages: 

The  single  lenses  (meniscus  and  meniscus  achromatic) 
must  be  used  with  a  relatively  small  stop,  which  means 
that  they  are  somewhat  slow.  They  are  fast  enough  for 
snapshots  in  good  light,  the  shutters  they  are  fitted  with 
being  adjusted  for  the  making  of  moderately  slow  "snaps". 
The  very  fact  that  they  require  a  small  stop  gives  them 
great  depth  of  focus,  however,  and  for  that  reason  errors  in 
focusing  are  largely  compensated  for,  resulting  in  a  high 
percentage  of  successful  pictures. 

The  Rapid  Rectilinear  Lenses  have  more  speed  than  the 
single  lenses,  and  are  also  better  for  architectural  work. 

The  Anastigmat,  f.6.3,  lenses  are  about  sixty  per  cent 
faster  than  the  Rapid  Rectilinear  lenses  and  are  corrected 
for  the  finest  definition  (sharpness).  When  used  at  their 
full  speed — that  is,  with  the  largest  opening — they  require 
accurate  focusing,  although  it  should  be  borne  in  mind  that 
both  the  length  of  focus  and  the  stop  opening  affect  this 
matter  of  depth  of  focus.  That  is  why  the  3A,the  largest  of 
the  Kodaks,  requires  more  accurate  focusing  than  the 
smaller  ones,  and  is  why,  when  we  get  down  to  the  Vest 
Pocket  size,  it  is  possible  to  use  an  Anastigmat  lens  with  a 
fixed  focus. 

An  Anastigmat  lens  does  not  require  any  more  accurate 
focusing  than  any  other  lens  when  used  with  the  same  stop. 
Take,  for  instance,  an  average  landscape  with  a  prominent 
object  in  the  foreground.  The  correct  stop  would  be/.16 
and,  if  the  sun  were  shining,  the  correct  exposure  1/25  of  a 
second.  This  same  stop  and  exposure  should  be  used 
with  a  Single  lens,  a  Rapid  Rectilinear  or  an  Anastigmat, 
and  the  depth  of  focus  with  the  same  focal  length  of  lens 
would  be  the  same  in  all  cases — no  more  accurate  focusing 
would  be  required  with  one  lens  than  with  another. 

But  when  the  light  isn't  very  good  and  an  Anastigmat 
is  used  at  its  full  opening,  or  nearly  its  full  opening,  in 
order  to  get  a  well  timed  snap  shot,  this  will  be  a  gain 
in  speed  but  a  loss  in  depth  of  focus.  The  object  at  the 
focused  distance  may  photograph  even  sharper  than  it 
would  with  the  Single  or  Rapid  Rectilinear  lenses,  but 
objects  a  little  nearer  the  camera  or  a  little  farther  away 

31 


FUNDAMENTALS  OF  PHOTOGRAPHY 

will  not  be  so  sharp  because  depth  of  focus  has  been  sacri- 
ficed for  speed.  And,  of  course,  this  same  thing  is  true  in 
using  a  large  stop  in  order  to  arrest  the  motion  of  moving 
objects.  With  a  fixed-focus  camera  working  at  a  fixed 
shutter  speed,  all  still  objects  at,  say,  fifty  feet  away, 
would  be  sharp  and,  with  a  good  light,  fully  timed,  but 
moving  objects  might  show  a  blur.  With  an  Anastigmat 
lens  opened  to/.6.3  and  a  shutter  speed  of  1/200  of  a  second, 
it  is  possible  to  arrest  moderately  fast  motion  and  get  a 
fully  timed  negative  (with  good  light),  but  in  such  case  care 
must  be  taken  to  focus  accurately. 


32 


CHAPTER  IV. 

THE  LIGHT  SENSITIVE  MATERIALS  USED  IN  PHOTOGRAPHY 

AS  was  explained  in  Chapter  I,  the  sensitive  coating  on 
films  and  papers  consists  of  bromide  or  chloride  of  silver 
held  in  a  thin  layer  of  gelatine,  and  thus,  photography  de- 
pends upon  the  fact  that  the  shiny,  white  metal  silver  when 
combined  with  certain  other  substances  forms  compounds 
which  are  sensitive  to  light  and  which  are  changed  in  their 
nature  when  they  are  exposed  to  light. 

Chemical  compounds  are  formed  by  the  combination  in 
definite  proportions  of  a  limited  number  of  elements,  of 
which  about  eighty  exist. 

These  elements  may  be  divided  into  the  two  classes  of 
metals  and  nonmefals,  and  the  metals  combining  with  the 
nonmetals  form  compounds  called  salts.  These  salts  are 
not  usually  formed  by  the  direct  combination  of  the  metal 
and  the  nonmetal  but  by  the  agency  of  acids. 


Fig.  35. 
Crystals  of  Silver  Nitrate. 

Thus,  the  first  step  in  making  a  light  sensitive  compound 
of  silver  is  to  dissolve  the  silver  in  nitric  acid.  When  silver 
is  put  in  nitric  acid,  it  is  dissolved  by  it,  and  if  the  solution 
is  dried  up  we  get  flat,  plate-like  crystals  of  silver  nitrate. 

33 


FUNDAMENTALS  OF  PHOTOGRAPHY 

These  crystals  of  silver  nitrate  dissolve  in  water  quite  easily, 
but  if  some  cooking  salt  solution  is  added  to  the  silver 
nitrate  solution,  the  silver  combines  with  one  of  the  com- 
ponents of  the  salt,  called  chlorine,  and  the  silver  chloride 
that  is  produced  is  not  soluble  in  water,  so  that  it  will  be 
visible  as  a  sort  of  white  mud  in  the  solution. 

Chlorine  is  one  of  a  group  of  elements  which,  because 
they  occur  in  sea  salt,  are  called  halogens,  from  the  Greek 
name  for  the  salt  sea.  Two  others  of  these  elements  are 
bromine  and  iodine,  and  the  silver  compounds  with  these 
three  elements  are  distinguished  by  their  extreme  insolu- 
bility in  water  and  their  sensitiveness  to  light.  Silver 
bromide  is  more  insoluble  than  silver  chloride  and  is  pale 
yellow  in  color;  silver  iodide  is  still  more  insoluble  and  is 
strongly  yellow. 

These  silver  compounds  are  formed  by  simply  adding  a 
solution  of  a  chloride  (such  as  cooking  salt),  bromide  or 
iodide  to  a  solution  of  silver  nitrate.  If  this  is  done  in  a 
water  solution,  the  silver  compound  will  settle  down  to  the 


• 1  \  C   ' 

WJM 

'v'^p^llJHra^ 

Fig.  36. 

Silver  Bromide  in  Suspension:  Left,  Without  Gelatine;  Right,  With 

Gelatine. 

Photograph  of  two  flasks  containing  silver  bromide  in  suspension.  The  flask  on  the 
left  shows  that  silver  bromide  without  gelatine  settles  to  the  bottom  of  the  solution 
The  one  on  the  right  shows  the  silver  bromide  held  evenly  in  suspension  by  gelatine. 

34 


LIGHT  SENSITIVE  MATERIALS  IN  PHOTOGRAPHY 

bottom  of  the  vessel,  but  this  may  be  prevented  by  adding 
to  the  water  some  gelatine,  like  that  used  for  cooking. 

The  gelatine  is  soaked  in  water,  and  then  when  it  is  swol- 
len it  is  dissolved  by  putting  it  in  warm  water  and  gently 
warming  and  shaking  until  it  is  all  dissolved.  Then  there  is 
added  to  this  the  right  quantity  of  bromide.  The  bromide 
dissolves  in  the  gelatine  solution  just  as  salt  would,  and  is 
stirred  up  to  get  it  evenly  distributed.  Meanwhile,  some 
silver  nitrate  has  been  weighed  out  so  that  the  right  amount 
is  taken  to  act  with  the  amount  of  bromide  chosen  and  is 
dissolved  in  water,  in  which  it  dissolves  very  easily.  This 
silver  nitrate  solution  is  then  added  slowly  to  the  bromide 
dissolved  in  the  gelatine,  and  produces  at  once  a  precipitate 
of  silver  bromide.  This  silver  bromide  is  sensitive  to  light 
so  that  before  adding  the  silver  nitrate  to  the  bromide  and 
gelatine  all  the  white  lights  are  turned  out  and  the  silver  is 
added  by  the  light  of  a  photographic  red  lamp. 

As  the  silver  is  added  a  little  at  a  time,  the  solution 
being  stirred  meanwhile,  the  gelatine  becomes  full  of  the 
smoothly,  evenly  precipitated  silver  bromide  distributed 
through  the  solution. 

If  the  emulsion  of  silver  bromide  in  gelatine  is  coated  on 
the  film  and  then  cooled,  the  gelatine  will  set  to  a  jelly,  still 
containing  the  silver  bromide  suspended  in  it,  and  then 
when  this  layer  is  dried,  we  get  the  smooth  yellowish  coat- 
ing, which  is  familiar  to  those  of  us  who  have  looked  at  an 
undeveloped  film  in  the  light. 

If  we  look  at  the  silver  bromide  film  through  a  very  high 
power  microscope,  we  shall  find  that  the  silver  bromide  is 
distributed  throughout  it  in  the  form  of  tiny  crystals.  These 
crystals  are  in  the  form  of  flat  triangular  or  hexagonal 
plates,  and  careful  investigation  has  shown  that  they  be- 
long to  the  regular  system  of  crystals.  When  these  crystals 
are  exposed  to  light,  no  visible  change  takes  place,  but  there 
must  be  some  change  because  when  a  crystal  of  silver  bro- 
mide, which  has  been  exposed  to  light,  is  put  into  a  devel- 
oper, the  developer  takes  the  bromine  away  from  the  silver 
and  leaves  instead  of  the  crystal  what  looks  under  a 
microscope  like  a  tiny  mass  of  coke,  which  is,  really,  the 
metallic  silver  itself  freed  from  the  presence  of  the  bromine. 

It  may  seem  strange  that  silver,  which  we  always  think 
of  as  a  bright,  shiny  metal  should  look  black,  but  when  it 
is  divided  up  in  this  irregular  way,  it  looks  black,  although 
it  is  the  same  thing  as  the  shiny  metal  we  are  familiar  with, 

35 


FUNDAMENTALS  OF  PHOTOGRAPHY 

just  as  a  black  lump  of  coke  is  the  same  thing  as  the  bright 
gleaming  diamond. 

If  the  silver  bromide  has  not  been  exposed  to  light,  then 
the  developer  has  no  power  to  take  away  the  bromine  from 


Fig.  37. 
Crystals  of  Silver  Bromide  before  (left)  and  after  (right) 
Development. 

The  photographs  above,  taken  through  a  very  powerful  microscope,  show  crystals  of 
silver  bromide  before  development  (on  the  left)  and  (on  the  right)  some  crystals  after 
they  have  been  changed  into  metallic  silver  by  development.  The  crystals  before 
development  are  transparent  except  where  they  are  seen  sideways  or  where  their 
edges  appear  darker.  After  development  the  clear  yellow  silver  bromide  is  turned  into 
a  black  coke-like  mass  of  silver  in  exactly  the  same  position  as  the  crystal  from  which 
it  was  formed. 

the  silver  and  leave  the  black  silver  behind,  so  that  we  see 
a  developer  is  a  chemical  that  has  the  power  to  take  away 
the  bromine  from  the  silver  in  a  grain  of  silver  bromide 
which  has  been  exposed  to  light  but  will  not  affect  one  which 
has  not  been  exposed  to  light. 

Wherever,  then,  the  light  in  the  Kodak  acts  upon  the 
silver  bromide  crystals  in  the  emulsion,  the  developer  turns 
them  into  black  grains  of  silver  and  we  get  an  image,  and 
whore  the  light  has  not  acted  the  developer  has  no  action 
and  no  image  is  produced.  The  chemical  part  played  by  a 
developer,  therefore,  is  the  freeing  of  the  metallic  silver  from 
the  bromine  associated  with  it. 

This  liberation  of  metals  from  their  compounds  is  the 
most  important  chemical  process  in  the  history  of  the  human 
race.  O". 

The  great  thing  which  has  distinguished  man  from  the 
•  alii!-  animals  has  been  his  ability  to  make  and  use  tools_^ 
and  weapons,  and  man  has  progressed  step  by  step  from  the 
earliest  days  when  he  used  a  flint  fastened  to  a  stick,  to  the 
present  time,  when  he  employs  the  marvelous  machinery 

36 


. 


\ 


LIGHT  SENSITIVE  MATERIALS  IN  PHOTOGRAPHY 

of  modern  civilization;  but  the  greatest  step  in  all  that  pro- 
gress came  when  men  found  out  how  to  get  metals  to  use  in 
the  place  of  stone.  All  the  earliest  weapons  were  made  of 
stone,  and  then  men  found  a  way  of  getting  tin  from  its 
ores,  and  found  that  when  this  tin  was  combined  with  cop- 
per, which  they  found  in  the  ground,  they  could  get  bronze, 
and  for  a  long  time  all  the  weapons  and  tools  were  made  of 
bronze,  and  then  came  the  greatest  discovery  of  all — they 
found  that  by  taking  iron  ore  and  heating  it  with  charcoal 
they  could  get  the  metal  iron,  which  made  such  beautiful 
tools  and  weapons;  and  from  the  time  that  men  found  out 
how  to  get  iron,  they  ceased  to  be  savages  and  began  to  be 
civilized. 

Iron  is  got  from  the  ore  by  heating  it  with  charcoal  or 
coke,  which  takes  away  the  other  components  of  the  ore 
and  leaves  the  metallic  iron  free.  Metals  can  be  got  out  of 
their  compounds  in  different  ways.  Quicksilver,  for  in- 
stance, can  be  got  by  merely  heating  its  oxide.  If  the  red 
oxide  of  quicksilver  be  heated  the  quicksilver  will  boil  off, 
and  can  be  collected  quite  pure  at  once.  Silver  is  rather 
easy  to  get,  and,  indeed,  if  we  take  a  solution  of  silver  nitrate 
and  add  some  iron  sulphate  to  it  the  metallic  silver  will  be 
thrown  out  as  a  black  sludge. 

The  developers  that  we  use  in  photography  play  the  same 
part  for  the  silver  that  the  charcoal  does  for  the  iron;  they 
take  away  the  bromine  from  the  silver  bromide  and  leave 
the  metallic  silver  behind. 

The  emulsion  coated  on  films  and  used  for  making  the 
negative  contains  silver  bromide  with  a  small  addition  of 
silver  iodide.  The  different  degrees  of  sensitiveness  are  ob- 
tained by  the  amount  and  duration  of  heat  to  which  the 
emulsions  are  subjected  during  manufacture,  the  most  sen- 
sitive emulsions  being  heated  to  higher  temperatures  and 
for  a  longer  time  than  the  slower  emulsions. 

If  a  slow  bromide  emulsion  is  coated  upon  paper,  the 
material  is  known  as  bromide  paper  and  is  used  for  printing 
and  especially  for  making  enlargements.  The  less  sensitive 
papers  which  are  commonly  used  for  contact  printing  by 
artificial  light,  contain  silver  chloride  in  the  place  of  silver 
bromide. 

Materials  which  are  to  be  used  with  development  must 
not  contain  any  excess  of  soluble  silver,  and  the  emulsion 
must  be  made  so  that  there  is  always  an  excess  of  bromide 
or  chloride  in  the  solution,  since  any  excess  of  soluble  silver 
will  produce  a  heavy  deposit  or  fog,  over  the  whole  of  the 

37 


FUNDAMENTALS  OF  PHOTOGRAPHY 

surface  as  soon  as  the  material  is  placed  in  the  developer. 
In  the  case  of  Solio  paper,  however,  which  is  not  used  for 
development  but  which  is  printed  out,  a  chloride  emulsion 
is  made  with  an  excess  of  silver  nitrate,  this  having  the 
property  of  darkening  rapidly  in  the  light,  so  that  prints 
can  be  made  upon  Solio  paper  without  development,  a  visi- 
ble image  being  printed  which  can  be  toned  and  fixed.  Solio 
paper  can  be  developed  with  certain  precautions,  but  only 
by  the  use  of  acid  developers  or  after  treatment  with 
bromide  to  remove  the  excess  of  silver  nitrate. 

In  the  early  days  of  photography  prints  were  usually 
made  on  printing  out  papers,  but  at  the  present  time  most 
prints  are  made  on  developing-out  chloride  and  bromide 
papers,  which  are  chemically  of  the  same  nature  as  the  nega- 
tive making  materials,  and  which  are  coated  with  emulsions 
containing  no  free  silver  nitrate. 


38 


CHAPTER  V. 

DEVELOPMENT. 

IN  chapter  IV  we  saw  that  the  chemical  process  of  devel- 
opment consists  of  the  removal  of  the  bromine  from  the 
silver  bromide  in  the  emulsion  so  as  to  leave  the  grains  of 
silver  behind. 

There  are  many  chemicals  which  will  remove  bromine 
from  silver  bromide  in  this  way,  but  in  order  to  act  as  a 
developer,  it  is  necessary  that  a  chemical  should  be  chosen 
which  has  the  power  of  turning  the  exposed  silver  bromide 
into  metallic  silver,  but  which  will  not  act  on  unexposed 
silver  bromide,  since,  if  the  developer  acted  on  the  unexpos- 
ed, as  well  as  on  the  exposed  grains,  we  should  not  get  an 
image  at  all,  but  the  whole  film  would  go  dark  when  put  in 
the  developer,  just  as  if  it  had  all  been  fogged  by  exposure 
to  light.  Only  a  very  limited  number  of  chemicals  have  this 
power  of  distinguishing  between  exposed  and  unexposed 
grains  of  silver  bromide  and,  consequently,  there  are  only 
a  few  substances  which  are  suitable  for  use  as  developers. 

The  chief  of  these  developing  substances  are  pyrogallol, 
or  "pyro"  as  the  photographer  calls  it,  hydroquinone  and 
elon,  all  of  which  are  chemically  related  to  aniline,  which  is 
used  as  the  base  of  coal  tar  dyes.  Hydroquinone  and  elon, 
indeed,  are  made  by  the  same  methods  as  those  used  for 
making  dyes,  but  pyro  is  made  by  distilling  gallic  acid, 
which  is  produced  by  fermenting  gall  nuts,  so  that,  although 
pyro  is  really  a  cousin  of  hydroquinone,  it  is  made  quite 
differently,  from  a  vegetable  product,  while  hydroquinone 
itself  is  made  from  aniline. 

Now,  if  we  take  a  solution  of  one  of  these  chemicals,  let 
us  say  pyro,  and  put  an  exposed  film  into  it,  we  shall  get  no 
development  at  all;  the  developing  agent  by  itself  having 
no  power  to  develop.  In  order  to  make  it  develop  we  must 
add  a  little  alkali  to  the  solution.  Any  kind  of  alkali  will 
make  it  develop,  but  the  most  convenient  one  to  use  is 
carbonate  of  soda  which,  in  its  crude  form,  is  called  sal-soda 

39 


FUNDAMENTALS  OF  PHOTOGRAPHY 

and  is  used  to  make  water  alkaline  for  washing.  If,  then, 
we  take  a  solution  of  pyro  and  add  some  sodium  carbonate 
to  it  it  will  develop  our  exposed  films;  but  a  solution  con- 
taining only  pyro,  carbonate  and  water  will  not  keep  and, 
if  .we  leave  it  in  the  air,  it  will  very  soon  darken  and  lose  its 
developing  power. 

In  order  to  make  it  keep,  there  is  added  to  the  developer 
some  sulphite  of  soda  because  the  developer  is  spoiled  by 
taking  up  oxygen,  and  sulphite  is  so  greedy  of  oxygen  that 
it  will  take  it  away  from  the  oxidized  pyro  or  take  it  in 
preference  to  the  pyro,  and  thus  protects  the  pyro  from  the 
oxidizing  action  of  the  air  and  enables  it  to  keep  its  develop- 
ing power,  although  the  sulphite  itself  has  no  developing 
power  at  all. 

The  essential  constituents  of  a  developer  therefore  are: 
The  developing  agent — pyro  or  hydroquinone  or  elon,  or 
Kodelon  which  is  a  relative  of  elon — the  alkali,  which  is 
generally  carbonate  of  soda,  and  the  preservative,  which  is 
sulphite  of  soda.  Very  often  a  developer  which  contains 
only  these  constituents  will  prove  difficult  to  handle.  It 
will  tend  to  give  fog,  that  is,  to  develop  unexposed  silver 
bromide  as  well  as  exposed  silver  bromide,  and  so,  in  order 
to  regulate  it,  there  is  put  in  a  little  potassium  bromide  to 
act  as  a  restrainer. 

The  various  developing  agents  behave  somewhat  differ- 
ently. Suppose,  for  instance,  that  we  make  up  two  devel- 
opers, one  with  hydroquinone  and  the  other  with  elon,  and 
start  to  develop  a  film  in  each  at  the  same  time.  In  the 
elon  developer  the  image  will  appear  very  quickly  on  the 
film  and  will  appear  all  over  the  film  at  the  same  time,  the 
less  exposed  portions  which,  of  course,  were  the  shadows  in 
the  picture,  appearing  at  the  same  time  as  the  highlights. 
On  the  other  hand,  with  the  hydroquinone  the  image  will 
appear  more  slowly,  and  the  most  exposed  portions,  or  the 
highlights  will  appear  first,  so  that  by  the  time  the  shadows 
have  appeared  on  the  surface  of  the  film  the  highlights  will 
have  acquired  considerable  density.  If  development  is  stop- 
ped as  soon  as  the  whole  image  is  out,  then  the  negative 
developed  in  elon  will  be  very  thin  and  gray  all  over,  while 
that  developed  in  hydroquinone  will  have  a  good  deal  of 
density  in  the  highlights.  Thus,  of  these  two  developers 
we  may  say  that  elon  gives  detail  first  and  then  slowly 
builds  up  density,  while  with  hydroquinone  the  detail  comes 
only  after  considerable  density  has  been  acquired.  It  is  for 
this  reason  that  these  two  developing  agents  are  used  in 

40 


DEVELOPMENT 

combination;  the  hydroquinone  gives  the  density  and  the 
elon  the  detail,  and  together  they  make  a  well  balanced 
v-  developer. 

These  differences  in  the  behavior  of  developing  agents 

fare  due  to  a  property  of  the  developer  which  can  be  ex- 
plained very  easily  by  an  analogy.    Suppose  that  we'  had 
Two^ automobiles  of  the  same  kind/  one  _of_20_horse  power ^ 

and  the  other  of  100  horse,power.<'Wriat  would  be  the  dif-  (  " 
ference  between  them?  Naturally,  the  high  horse  power  . 
automobile  would-be  able  to  go  faster  than  the  other;  but 
in  a  city,  at  any  rate,  either  of  them  would  be  able  to  go 
as  fast  as  was  safe,  and  no  one  would  wish  to  use  the  higher 
horse  power  for  increased  speed;  but  the  advantage  of  the 
high  horse  power  would  be  found  whenever  the  automo- 
biles were  used  against  adverse_cirgumstances,  as,  for  in- 
stance, against  "high"  winds,  TrPsnow,  or  in  climbing  hills,  C 
when  the  high-power  machine  would  be  able  to  keep  up  its 
speed  against  the  difficulties,  and  the  lower  power  machine 
would  be  slowed  and  might  even  be  unable  to  get  ahead. 
The  difficulties  which  affect  development  in  a  manner  cor- 
responding to  the  effect  of  hills  or  winds  for  an  automobile 
are  cold  and  bromide.  The  addition  of  bromide  has  the 
same  effect  on  a  developer  that  a  hill  has  on  an  automobile 
— it  slows  it  down;  but  bromide  has  far  more  effect  on 
a  low  power  developer  like  hydroquinone  than  it  has. on  a 
high  power  developer  like  elon;  the  effect  of  bromide  on 
elon  is  very  small,  while  on  hydroquinone  it  is, very  great. 
In  the  same  way,  hydroquinone  develops  very  slowly  when 
it  is  cold,  while  elon  is  not  nearly  so  much  affected  by  tem- 
perature. ( 

The  analogy  between  the  horse  power  of  the  automobile 
and  theipower  of  the  developer  is  really  very  close.  The 
high  horse  power  automobile  will  start  from  rest  very  much 
more  quickly  than  the  machine  of  lower  horse  power,  just 
as  the  elon  developer  forces  out  the  image  all  over  the  film 
much  more  rapidly  than  the  hydroquinone  developer.  Just 
as  the  horse  power  of  an  automobile  could  be  measured  by 
the  effect  of  a  hill  on  its  speed  so  thepo'wer  of  a  developer 
can  be  measured  by  the  reduction  of  density  produced  by 
the  addition  of  bromide,  and  just  as  one  would  not  wish  to 
have  an  over-powered  automobile,  hard  to  handle  and  al- 
ways picking  up  speed  very  rapidly,  so  it  is  difficult  to  use 
the  very  high  power  developers,  and  elon,  for  instance,  is 
rarely  used  alone,  but  is  generally  adjusted  by  admixture 
with  the  slower  hydroquinone. 

4i 


FUNDAMENTALS  OF  PHOTOGRAPHY 

Pyro  is  an  almost  ideal  developer  for  negative  making, 
but  owing  to  the  fact  that  the  pyro  is  changed  during  devel- 
opment into  a  yellow  colored  substance,  some  of  which  re- 
mains with  the  silver  in  the  image,  pyro  tends  to  give  a 
slightly  yellowish  or  brownish  image.  The  yellowish  stain 
is  prevented  from  forming  by  sulphite,  so  that  the  more 
sulphite  there  is  in  a  developer  the  less  tendency  to  warmth 
the  deposit  will  show,  but,  nevertheless,  pyro  is  not  used 
for  papers,  for  which  the  blue-black  image  obtained  with 
elon  and  hydroquinone  is  preferred. 

When  a  film  is  developed,  it  is  only  the  grains  of  silver 
bromide  which  have  been  changed  by  the  action  of  light 
that  are  affected  by  the  developer.  The  grains  that  have 
not  been  changed  are  not  affected;  at  the  beginning  of 
development  there  are  a  great  many  exposed  grains  ready 
to  be  developed,  and  then  as  development  proceeds,  these 
exposed  grains  are  turned  into  grains  of  black  silver,  so  that 
the  number  of  developable  grains  decreases  during  develop- 
ment until  at  last  there  are  no  developable  grains  left;  all 
those  which  can  be  developed  have  been  acted  upon,  and 
,    .development  reason _ 

The  rate  at  which  the  development  proceeds  can  best  be 
1  understood  by  an  analogy  from  fishing.  Suppose  one  went 
out  fishing  and  found  a  pond  where  nobody  had  ever  fished- 
and  there  were  about  four  hundred  fish  in  the  pond.  In  the 
first  day's  fishing  one  might  catch  half  the  fish  in  the  pond, 
or  two  hundred  fish,  but  the  second  day  one  would  not  ex- 
pect to  catch  the  other  half;  all  one  could  expect  to  catch 
would  be  the  same  proportion  of  the  remaining  fish,  that  is, 
half  of  what  were  left,  or  one  hundred  fish,  and  the  third 
day  one  might  catch  half  of  what  were  left  again,  or  fifty 
fish,  and  the  fourth  day  half  of  what  were  left  again,  or 
twenty-five  fish,  and  so  on,  the  catch  growing  smaller  as 
the  number  left  decreased,  until  finally  no  fish  were  left  to 
catch,  or  more  probably  until  one  got  tired  of  trying  to  get 

jjip  fr>w  rgmjining  fish.    2&-^ 

This  is  what  happens  in  development.  The  rate  at  which 
the  grains  develop  depends  upon  the  number  of  undevelop- 
ed grains  left,  and  as  the  grains  are  developed  up  and  the 
number  of  undeveloped  grains  remaining  become  less,  fewer 
and  fewer  grains  develop  in  each  minute,  until  finally,  it  is 
not  worth  while  to  prolong  the  development  in  order  to 
get  any  more  density.    (See  Fig.  -££.)4  3 

If  the  development  is  prolonged  beyond  the  point  at 
which  all  the  exposed  grains  are  developed,  then  there  is  a 

42 


s- 


:> 


DEVELOPMENT 

danger  of  developing  some  of  the  unexposed  grains,  which 
produces  a  veil  over  the  whole  picture — exposed  and  un- 
exposed portions  alike — and  this  veil  is  known  as  fog. 


At  Beginning 


After 
2  Minutes 


After 
1  Minute 
Development  of  exposed  grains  in  a  film  which  is  half  developed  in  one  minule. 


After 
3  Minutes 


After 
4  Minutes 


After 

5  Minutes 


Fig.  38. 


The  growth  of  the  image  during  development  is  referred 
to  as  a  growth  of  density,  that  is  to  say,  the  density  is  a 
measure  of  the  number  of  grains  of  silver  which  are  produced 
at  any  given  point  because  these  grains  of  silver,  after  the 
film  has  been  cleared  by  the  fixing  bath,  obstruct  the  passage 
of  light  through  the  film.  The  density  of  an  image  is  meas- 
ured in  units  which  are  based  on  the  amount  of  silver  which 
will  let  through  l/10th  of  the  light,  so  that  if  only  l/10th  of 
the  light  falling  on  the  negative  gets  through  a  certain  part 
of  it,  that  portion  of  the  negative  is  said  to  have  a  density 

43 


FUNDAMENTALS  OF  PHOTOGRAPHY 


of  1.  The  blackest  part  of  a  negative  may  have  a  density 
of  perhaps  2,  the  middle  tones  1  or  less,  and  the  shadows, 
perhaps  l/10th.    (Fig.  39.) 

The  difference  of  density  between  the  darkest  portion 
and  the  lightest  portion  of  the  negative  is  called  its  contrast. 
In  most  negatives  the  shadows  are  nearly  clear  so  that  the 
contrast  depends  chiefly  on  the  density  of  the  darkest  por- 
tion, but  this  is  not  necessarily  so  because  an  over-exposed 
negative,  or  one  taken  of  a  very  flat  subject,  may  have  no 
clear  portion  in  it  and  may  be  even  very  dense  owing  to 
over-exposure,  and  yet  not  contrasty  at  all  because  there  is 
very  little  difference  between  the  density  of  the  most  ex- 
posed portion  and  that  of  the  least  exposed  portion,  the 
negative  being  very  dense  all  over.  It  is  necessary  to  keep  >, 
learly  in  mind  this  difference  between  the  density  and  the^ 
contrast. 

Since  the  contrast  depends  chiefly 
upon  the  density  of  the  highlights, 
it  grows  during  development  just  as 
the  density  does.    It  grows  rapidly 
at  first,  when  there  are  many  grains 
to   be   devel- 
oped, and  then 
more  slowly 
until,  finally, 


D&nsiTy  z> 


/Density 


Shadows 

Fig.  39. 


Half  Tones  Highlights 

Densities  of  Various  Parts  of  a  Negative.-* . ) 


when  the  grains  are  all  developed,  the  negative  will  not 
give  any  more  contrast  however  long  development  may  be 
prolonged,  and  a  continuation  of  development  will  only 
result  in   the  production  of  fog.    (Fig.  -It).) 


Aftei    I   Minute  After  2  Minutes  After  3  Minutes 

Fig.  40.    Growth  of  Contrast  During  Development. 

44  /      Ah^- 


DEVELOPMENT 


The  final  contrast  which  can  be  obtained  depends  upon  the 
kind  of  emulsion  used.  The  fast  emulsions,  such  as  the 
film  emulsions,  give  moderate  contrast,  but  the  slow  emul- 
sions, such  as  those  used  for  copying  purposes  or  for  making 
lantern  slides,  are  specially  made  to  give  great  con- 
trast when  development  is  prolonged.    (Fig.  41.) 


Highly  Sensitive 


Medium  Sliced  Plate  -  Slowi 

Fig.  41. 
Greatest  Contrasts  With  Different  Emulsions. 


It  would  be  convenient  if  the jaaratfactiHior  could  make 
the-kim  so  that  it  would  be  impossible  to  over-develop  it, 
but  this  is  not  practicable.  It  would  be  possible  if  a-£k» 
developed  at  an  even  rate  and  then  stopped  developing 
when  it  was  correctly  developed  as  is  shown  in  Fig)  42, 
where  development  is  supposed  to  go  straight  on  for  a 
given  time  and  then  stop  altogether,  the  JiBrnot  changing 
after  that  time.  But  the-#i»  does  not  develop  like  this; 
the  growth  of  the  image  gets  slower  as  time  goes  on  but  it 
takes  a  very  long  time  indeed  to  stop  completely,  so  that 
the   growth  of  the  image  occurs  as  shown  in  Fig.  43.    If 


w 


TIME  OF  DEVELOPMENT 

Fig.  42. 
Growth  of  Image  During  Development 
(to  be  desired)  i 


TIME   OF   DEVELOPMENT 


Fig.  43. 

Growth  of  Image  During  Development 
(actual) 


45 


< 


c^^ 


J- 


FUNDAMENTALS  OF  PHOTOGRAPHY 

a  film  were  made  so  that  we  had  to  develop  it  as  far  as  we 
could,  it  would  take  too  long  to  develop,  and  therefore  it  is 
necessary  to  make  a  film  that  is  capable  of  giving  more 
density  than  is  required  in  order  that  it  may  be  developed 
in  a  sufficiently  short  time;  this  means  that  we  must  be 
able  to  stop  development  at  the  right  time  to  get  enough 
density  and  contrast,  the  density  being  the  blackness  of 
the  image  and  the  contrast. 

01d-timfe'pi»#tegPBpiM»g  used  to  take  pride  in  the  accuracy 
with  which  they  could  judge  the  progress  of  the  develop- 
ment ml  Oftjjiiti  i  r  t  and  it  was  regarded  as  quite  wrong  when, 
in  recent  years,  people  insisted  that  nagafeiitee  could  be 
developed  just  as  well  by  timinp-  the  development  as  by 
watching  it,  and  that  it  was  better  tor  the  aapMnto  not  to 
be  watched.   /yxrv^A-     A  o^jl   ,/-£-  o^o 

The  customary  way  of  judging  the  progress  of  develop- 
ment m  a  aapttite  is  to  hold  it  up  to  a  lamp  and1  look    ' 
through  it/ but  unless  one  has  had  a  lot  of  experience  he  is 
very  likely  to  be  cteTeived  because  the  apparent  density  M 
a  negative  Jiolduip  fciwurirgli  t  is  very  difficult  to  judge.    The  '' 
which  has  not  been  developed  makes  it  appear 


nger   than   it   really,  is,   and   beginners  almost  always  / 
under-develop  rreprtiwi^if  they  try  to  judge  when  to  sto^y 

development,  [[for  some  reason  it  is  necessary  to  judge 
the  progress  of  development  by  inspection  (and  this  applies 
particularly  to  Im&mm**iUtm) ,  the  best  way  is  to  turn  the' 
emulsion  side  to  the  light  and  look  through  from  the  bark. 
This  is  much  less  misleading  than  if  they  are  examined  from 
the  front. 

T4icro  in  no  doubt,  however,  that  j-rre-bcst-  method  of 

judging  development  is  simply  to  develop  for  a  fixed  time.  - 

j 

are  best  developed  in  a  fiJniXamk.  and  the  time  of  e 


development,  at  a  temperature  of  65°  imml&m&mlmdH&iimtprr , 
is  20^minuteo^?jrhis,time  depends  on  the  temperature.  It 
the  temperature  is  lower  than  65°  the  time  must  be  in- 
creased, and  if  it  is  higher  than  65°  the  time  must  be  reduced. 


Instructions  for  development  are  furnished  with  each 
Kodak  or  Premo  tank.  It  might  be  thought  that  if  the 
£JajF  were  over-exposed  and  so  gave  density  easily  it  should 
be  developed  for  a  shorter  time  than  if  it  had  received  less 
exposure,  but  this  idea  is  quite  wrong,  because  what  is 
wanted  in  a  JMpative-  is  not  correct  density,  w&m&pimW 
.ill.et-  the  time  of  printing,  but  correct  contrast,  and  the 
contrast  is  controlled  by  the  time  of  development.    An  over- 

46 


DEVELOPMENT 


/ 


exposed  ftiw^will  tend  to  have  too  little  contrast,  and  if  the 
development  is  lessened  the  contrast  will  be  still  further  re- 
duced and  the  nognifave  will  be  fiat.  On  the  other  hand,  an 
under-exposed  titiBt  tends  to  be  too  contrary,'  and  must  not 
be  forced  in  development  or  it  may  be  unprintable,  and  so 
whatever  the  exposure,  the  best  result  will  be  obtained  by 
the  use  of  the  normal  time  of  development.  Of  course,  the 
best  imgmtmm  can  only  be  obtained  by  correct  exposure  as 
well  as  by  correct  development,  and  it  is  a  mistake  to  think 
that  we  can  correct  errors  in  exposure  by  deviation  from 
the  correct  time  of  development. 


i.> 


y^r^s^ 


47 


CHAPTER  VI. 


THE  STRUCTURE  OF  THE  DEVELOPED  IMAGE. 

THE  silver  grains  which  form  the  developed  image  are 
held  in  a  layer  of  gelatine.  This  gelatine  is  used  in 
making  the  emulsion  which  is  coated  on  the  support  to  make 
the  sensitive  film. 

Gelatine  is  a  very  interesting  substance,  and  its  charac- 
teristics are  markedly  different  from  those  of  most  other 
chemical  substances.  Most  chemical  substances  form  crys- 
tals, and  many  of  them  are  soluble  in  water.  When  they 
are  dissolved  in  water,  the  solution  is  quite  homogeneous, 

that  is  to  say,  alike  in  its 
properties  in  all  its  parts. 
Substances  generally  will 
dissolve  in  water  to  a  fixed 
extent,  dependent  on  the 
temperature.  We  say  of  one 
material,  for  instance,  that 
it  is  soluble  to  the  extent  of 
30%,  meaning  that  a  hun- 
dred parts  of  water  will  take 
up  30  parts  of  the  material. 
If  we  heat  the  solution  it  will 
usually  dissolve  more,  but 
then  when  it  cools  again 
the  material  will  crystallize 
out  so  that  whatever  we  do 
we  can  only  obtain  the  fixed 
30  parts  per  hundred  re- 
maining in  solution. 

Gelatine  behaves  quite  dif- 
ferently to  this.  In  cold 
water  it  does  not  dissolve 
but  it  swells,  as  if,  instead  of  the  gelatine  dissolving  in  the 
water,  the  water  dissolves  in  the  gelatine.  If  the  water  is 
heated,  the  gelatine  will  dissolve  in  it,  and  it. will  dissolve 
to  any  extent.  You  cannot  say  that  there  is  a  definite  solu- 
bility  of  gelatine  in  water.    The  more  gelatine  is  added,  the 

48 


Fig.  44. 
Swelling  of  Gelatine  Cube. 


STRUCTURE  OF  THE  DEVELOPED  IMAGE 


thicker  the  solution  becomes,  but  there  is  no  point  at  which 
the  gelatine  will  refuse  to  dissolve. 


Fig.  45. 
Reticulation. 

If  we  heat  a  gelatine  solution  it  will  become  thinner  and 
less  viscous  when  hot,  and  will  not  recover  completely  when 
cool;  it  will  remain  thinner  than  if  it  had  not  been  heated, 
so  that  the  heating  of  the  gelatine  solution  produces  a  per- 
manent change  in  its  properties.  If  we  cool  a  gelatine  solu- 
tion, the  gelatine  will  not  separate  from  the  solution  in  a 
dry  state,  but  the  whole  solution  will  set  to  a  jelly,  which 
we  might  consider  a  solution  of  water  in  the  gelatine.  If  we 
heat  the  jelly  it  will  melt  again,  and  we  can  melt  and  reset 
a  jelly  many  times,  but  in  doing  so  we  shall  produce  a  pro- 
gressive change  in  the  jelly,  and  if  we  continue  the  process 
too  long,  sooner  or  later  it  will  refuse  to  set  and  will  remain 
as  a  thick,  gummy  liquid. 

Gelatine  belongs  to  the  class  of  substances  which  are 
called  colloids,  the  name  being  derived  from  a  Greek  word 
meaning  gummy. 

When  a  gelatine  jelly  is  dried,  it  shrinks  down  and  forms 
a  horny  or  glassy  layer  of  the  gelatine  itself,  smooth  and 
rather  brittle,  and  this  dry  gelatine  when  placed  in  water 
will  at  once  absorb  the  water  and  swell  up  again  to  form  a 
jelly. 

49 


FUNDAMENTALS  OF  PHOTOGRAPHY 


An  interesting  and  important  property  of  the  drying  and 
swelling  of  gelatine  is  that  it  swells  almost  entirely  in  one 
direction,  namely,  that  in  which  it  was  dried.  This  is  illus- 
trated in  Fig.  44.  In  this,  A  represents  a  small  cube  cut  out 
of  a  sheet  of  gelatine  which   was  originally  dried   in   the 

horizontal  plane  when  it  was 
made.  If  this  cube  is  placed 
in  water,  it  will  not  swell  in 
all  directions,  becoming  a 
bigger  cube,  but  it  will  swell 
almost  entirely  in  the  direc- 
tion in  which  it  dried  down, 
and  will  take  the  form  B 
and,  finally,  the  form  C. 

The  explanation  of  this 
directional  swelling  of  the 
gelatine  jelly,  and  also  of  the 
fact  that  gelatine  solutions 
change     permanently     with 


Fig.  46. 

Spot  on  Gelatine 

Caused  by  Moisture. 


heating,  lies  in  the  fact  that  gelatine  is  not  a  uniform  sub- 
stance but  has  an  internal  structure.  Probably,  gelatine  has 
a  structure  somewhat  like  that  of  a  sponge,  but  the  structure 
is  very  small  and  has  not  the  elasticity  of  the  sponge. 

When  the  gelatine  is  in  the  jelly  state,  it  is  as  though  the 
sponge  were  full  of  water,  and  then  it  is  fairly  rigid,  because 
of  the  water  contained  in  the  pores.  When  the  water  is 
dried  out,  the  sponge  structure  shrinks  down,  and  if  it  is 
stretched  out  in  one  direction  by  being  coated  on  film  or 
paper,  for  instance,  it  will  shrink  down  vertically  just  as  a 
sponge  without  elasticity  would  fall  into  a  flat  mass  if  placed 
on  the  table. 

When  the  gelatine  solution  is  heated  and  the  gelatine 
dissolves,  it  seems  at  first  to  retain  a  certain  amount  of  its 
structure,  as  if  the  sponge  had  disintegrated  and  was  dis- 
tributed through  the  solution  but  the  sponge  structure  had 
not  entirely  disappeared.  Then,  if  the  temperature  is  raised, 
it  behaves  as  if  the  structure  were  slowly  breaking  up  and 
dissolving,  so  that  after  a  considerable  heating  at  a  high 
temperature  the  whole  solution  becomes  homogeneous. 
When  this  solution  is  cooled  and,  finally,  set  to  a  jelly,  it 
has  to  re-establish  a  new  sponge  structure,  and  this  will  be 
different  to  the  original  one  and  probably  of  less  strength. 

This  explanation  of  the  behavior  of  gelatine,  th.it  it  has 
an  internal  structure  which  can  persist  even  in  solution, 
seems  to  account  for  most  of  its  properties  and  behavior. 

?o 


STRUCTURE  OF  THE  DEVELOPED  IMAGE 

When  a  gelatine  jelly  contains  only  such  an  amount  of 
water  that  it  still  contains  a  considerable  proportion  of 
gelatine,  over  10%  for  instance,  the  jelly  will  be  strong  and 
tough,  but  if  the  jelly  contains  much  less  gelatine  than  this, 
it  will  be  weak  and  likely  to  rupture  on  any  kind  of  strain. 
This  is  a  very  important  matter  in  dealing  with  photo- 
graphic films.  When  the  film  is  first  placed  in  the  developer 
the  gelatine  at  once  commences  to  swell.  As  long  as  it  does 
not  swell  too  much  it  is  easily  handled,  but  if  it  swells  too 
far,  then  it  becomes  very  tender  and  is  likely  to  be  damaged 
by  touch,  and  in  extreme  cases  will  swell  so  much  that  it 
will  loosen  from  its  support  or  wrinkle  up  in  what  is  called 
"reticulation". 

The  swelling  of  a  gelatine  film  is  influenced  by  the  tem- 
perature of  the  solution  in  which  it  is  placed  and  also  by  the 
presence  of  other  substances  in  the  solution.  A  small 
amount  of  either  acid  or  alkali  will  produce  a  considerable 

increase  in  the  swel- 
f*C~       ~^/A  ling,   and   since   the 

W/'-'r  \\\  developer  is  alkaline 


and  the  fixing  bath 
is  acid ,  both  these  so- 
lutions  have  a  great 
tendency  to  swell 
the  gelatine,  especi- 
ally when  they  are 
warm.  On  the  other 
hand,  sulphites  tend 
to  prevent  swelling, 
so  that  an  increase 
in  the  concentration 

^r-^     ^— ^. ., ,,.,,,,  .v  ,,„,,. ,  of  the  sulphite  in  a 

developer    or    fixing 

bath  will  diminish  it. 

An  even  greater  aid 
in   preventing  swel- 

^.  .^ ,,  .  .  ,  _     ling  is  the  hardener 

in  the  fixing  bath. 
The    hardening 


Fig.  47.  agents  used  in  fixing 

The  Way  a  Waterspot  Dries.  baths  are  the  alums, 

which  not  only  pre- 
vent the  swelling  of  the  gelatine  temporarily  but  which  per- 
manently harden  the  structure  of  the  gelatine  so  that  it  will 
not  easily  swell.    It  is  for  this  reason  that  the  alum  is  in- 

51 


FUNDAMENTALS  OF  PHOTOGRAPHY 

troduced  into  the  fixing  bath  so  that  after  fixing  the  film  will 
not  become  soft  and  disintegrate  in  washing. 

Reticulation  is  due  to  local  strains  in  the  gelatine,  and  a 
sudden  change  in  the  temperature  of  solutions  will  some- 
times produce  this  effect.  If  a  film  is  transferred  for  instance 
from  a  cold  fixing  bath  containing  a  hardener  to  very  warm 
wash  water,  the  whole  film  will  sometimes  pucker  into  tiny 
reticulations,  a  good  example  of  which  is  shown  in  Fig.  45. 
If  one  part  of  the  film  contains  much  more  moisture  than 
another,  the  silver  image  itself  is  liable  to  become  distorted 
by  the  movement  of  the  gelatine,  and  of  the  silver  grains  in 
it.  If  a  drop  of  water,  for  instance,  falls  on  a  film  and  this 
is  dried  rapidly,  it  will  often  produce  a  curious  ring-shaped 
mark,  the  middle  of  the  drop  being  lighter  and  the  edge  of 
the  drop  darker  than  the  surrounding  negative,  Fig.  46. 
The  explanation  of  this  is  shown  in  Fig.  47.  The  gelatine 
swells  up  where  the  spot  of  water  fell  on  it,  and  as  it  dries 
again  a  strain  is  produced  by  the  collapse  of  the  center  of  the 
swollen  spot,  and  so  the  gelatine  and  silver  grains  are  pulled 
in  to  the  edges  of  the  spot  and  there  produce  the  dark  ring. 


APPEARANCE 

OF 

DEVELOPED 

EMULSION 

WHEN 
MAGNIFIED 


■400  diameters 


100  ilia  meters 

900  diameters 


Fig.  48. 

Appearance  of  Emulsion  After  Development  When  Magnified. 

The  developed  image  consists  of  grains  of  silver,  each 
grain  under  sufficient  magnification  looking  like  a  little  mass 

52 


STRUCTURE  OF  THE  DEVELOPED  IMAGE 


of  coke,  replacing  one  of  the  silver  bromide  crystals  which 
were  originally  formed  in  the  emulsion  and  keeping  the 
same  position.  See  Fig.  37.  When  we  look  at  a  negative  it 
appears  perfectly  smooth  to  the  eye,  but  under  a  small 
degree  of  magnification  it  begins  to  show  an  appearance  of 
graininess. 

It  must  not  be  thought,  however,  that  with  a  magnifying 
glass  we  can  see  the  silver  grains  themselves.  The  silver 
grains  are  so  small  that  to  make  them  visible  requires  power- 
ful magnification.  What  we  see  through  the  magnifying 
glass  are  clumps  of  grains. 

Suppose  that  an  aviator  is  flying  over  country  dotted  with 
occasional  woods  and  clumps  of  bushes.  If  he  is  flying  near 
to  the  ground,  he  will  be  able  to  distinguish  the  separate 
trees  and  bushes.  If  he  goes  higher,  he  will  no  longer  be 
able  to  see  them  separately  but  he  will  see  them  in  little 
clumps  of  two  and  three  where  they  are  close  together  with 

the  spaces  where  they  are 
^r'y^r'^^if^jyt^'^r-^yh.  farther  apart  showing  be- 
•  9~  '  /*"> *••'"'■  '  **  •'*.    **\*«u      tween  them,  and  then  as  he 

goes  higher  still,  he  will  no 
longer  be  able  to  see  these 
small  clumps,  but  will  be 
able  to  see  only  the  large 
masses  of  woodland  or  for- 
est. In  the  same  way  when 
we  look  at  a  negative  under 
a  low  magnification,  we  see 
the  larger  masses  of  clumps 
of  grains,  and  then  as  we 
increase  the  magnification 
we  see  the  smaller  clumps 
of  grains,  and  then  finally 
at  a  very  high  magnifica- 
tion we  see  the  grains  them- 
selves, Fig.  48. 

These  clumps  of  grains 
which  we  can  see  under  low 
magnification  are  made  up 
of  grains  which  are  not  all 
in  the  same  layer.  This  can 
be  seen  by  first  of  all  photographing  an  image  from  above 
and  then  cutting  a  section  down  through  it  so  as  to  see  how 
the  grains  lie  one  below  the  other.  In  Fig.  49  A  it  will  be 
seen  that  the  image  is  as  much  as  six  grains  deep  so  that 

53 


Vertical  Section  Showing  Grain 
Deposit. 


Horizontal  Plan  of  Same 
Grain  Deposit. 


Fig.  49. 


FUNDAMENTALS  OF  PHOTOGRAPHY 


Exposed  1  Unit  of  Time 


Exposed  16  Units  of  Tin 


Exposed  4  Units  of  Time. 


Fig.  50. 


Exposed  64  Units  of  Time. 


many  of  the  clumps  of  grains  seen  in  Fig.  49B  are  not  made 
up  of  grains  in  the  same  layer  but  of  grains  in  different 
layers,  some  on  the  top  and  some  below  . 

The  distribution  of  the  grains  in  the  depth  of  the  film  is 
interesting.  It  might  be  thought  that  with  short  exposures 
the  image  would  be  on  the  top  of  the  film  and  that  as  the 
exposure  was  continued,  the  light  would  penetrate  farther 
and  farther  into  the  film,  making  the  grains  in  the  lower 
layers  more  and  more  developable.  This  sometimes  seems 
to  be  the  case,  but  with  some  emulsions  it  is  not  so,  as  is 
proved  by  the  photographs  of  sections  shown  in  Fig.  50, 
which  are  cut  from  an  N.  C.  film.  These  are  fully  developed 
so  that  the  effect  of  development  is  eliminated,  and  they 
show  that  the  grains  are  exposed  at  all  parts  of  the  film  to 


VTV^* N-x>T->?*  -"-^^V^?!*-."^  7"i 


Fig.  51. 
Showing  Progress  of  Development  from  Surface  to  Base  of  Emulsion. 

54 


STRUCTURE  OF  THE  DEVELOPED  IMAGE 

an  almost  equal  extent,  though  in  the  second  and  third 
prints  there  is  a  slight  tendency  for  the  image  to  be  more 
on  the  top  of  the  film.  It  looks  as  though  the  emulsion  con- 
tains grains  of  various  degrees  of  sensitiveness  and  the  more 
sensitive  grains  are  made  developable  first.  Further,  since 
there  is  certainly  more  light  at  the  surface  of  the  film,  it 


mm,.*M>v<^^?^ia&^w?^^'^-f^i*1'''!::~''' 


Strong  or  Concentrated  Developer. 

A 

Weak  or  Diluted  Developer. 
Fig.  52. 

must  be  a  fact  that  the  more  sensitive  grains  are  found  in 
the  lower  parts  of  the  film. 

During  development,  however,  there  is  an  appreciable 
effect  due  to  the  penetration  of  the  developer  into  the  film. 
This  is  shown  in  Fig.  51,  where  it  is  seen  that  at  the  be- 
ginning of  development  only  the  surface  of  the  emulsion  is 
developed,  and  then  as  development  continues  the  develop- 
er penetrates  into  the  film  and  develops  more  and  more 
deeply  in  it.  In  the  case  of  a  strong  developer  this  effect  is 
accentuated,  because  a  strong  developer  will  develop  the 
surface  to  good  density  before  it  has  penetrated  through 
the  emulsion,  while  a  weak  developer  will  penetrate  at  the 
same  rate  as  the  strong  developer  and  will  not  develop  so 
rapidly,  so  that  with  a  strong  developer  there  is  a  tendency 
for  the  image  to  be  confined  to  the  surface  of  the  emulsion, 
and  with  a  weaker  developer  for  it  to  penetrate  through  the 
whole  emulsion.  This  effect  is  well  shown  in  Fig.  52,  where 
two  photographs  are  shown  of  the  edge  of  an  exposed  image, 
the  image  being  shown  as  the  dark  part  on  the  left,  while  on 
the  right  we  have  the  light  deposit  of  grains  due  to  fog.  In 
the  upper  picture,  the  image  was  developed  with  a  very 
strong  developer,  while  in  the  lower  picture  it  was  devel- 
oped with  a  much  weaker  developer,  and  it  will  be  noted 
that  the  weak  developer  has  penetrated  right  through  the 
image  to  the  back,  while  with  the  strong  developer  the  image 
has  not  developed  through  to  the  back  of  the  film,  although 
care  was  taken  to  develop  the  images  to  the  same  apparent 
density. 

55 


FUNDAMENTALS  OF  PHOTOGRAPHY 

There  is  a  curious  effect  shown  in  these  photographs  at 
the  point  marked  A,  where  it  is  seen  that  at  the  edge  of  the 
developed  image  the  fog  grains  are  not  developed  in  the 
lower  part  of  the  film;  it  is  as  if  they  had  been  eaten  away. 
There  is  no  doubt  that  the  reason  for  this  is  that  the  bro- 
mide liberated  during  development  of  the  heavy  image  has 
prevented  the  fog  grains  close  to  the  edge  of  the  image  from 
developing.  In  extreme  cases  this  will  sometimes  surround 
a  dense  image  with  a  white  line. 


56 


CHAPTER  VII. 

THE  REPRODUCTION  OF  LIGHT  AND  SHADE  IN 
PHOTOGRAPHY. 

PHOTOGRAPHY  is  the  art  of  making  representations  of 
natural  objects  by  mechanical  and  chemical  processes. 
These  representations  deal  with  differences  of  brightness, 
color  being  ignored,  except  in  color  photography,  and  the 
object  of  the  photographic  process  is  to  translate,  as  ac- 
curately as  possible,  the  degrees  of  brightness  which  occur 
in  natural  objects  into  corresponding  degrees  of  brightness 
in  a  photographic  print. 

It  is  not  possible  to  convey  any  impression  in  a  photo- 
graph of  the  brightness  of  an  object  of  even  brightness;  a 
piece  of  black  velvet  seen  in  bright  sunlight  is  brighter  than 
a  piece  of  white  paper  in  a  dark  room,  so  that  it  is  impos- 
sible to  speak  of  the  brightness  of  paper  or  the  blackness  of 
velvet  unless  there  is  some  standard  of  comparison  by  which 
it  can  be  measured.  If  black  marks  are  made  on  the  white 
paper  and  then  photographed,  the  resulting  print  will  re- 
produce the  relative  intensity  of  the  black  marks  and  of 
the  white  paper. 

When  a  representation  of  a  natural  object  is  made  on  a 
flat  surface,  the  form  can  be  represented  only  by  differences 


Fig.  53. 
Two  Tones. 


Fig.  54. 
Three  Tones. 


57 


FUNDAMENTALS  OF  PHOTOGRAPHY 


of  brightness  or  color.  Shape  is  only  possible  in  sculpture. 
The  painter  uses  differences  of  brightness  and  of  color, 
while  the  black  and  white  draftsman  uses  only  the  differ- 
ences of  brightness.  Except  in  the  special  branch  of  color 
photography,  photographs  deal  only  with  the  reproduction 
of  objects  in  their  degrees  of  brightness. 

The  different  degrees  of  brightness  are  spoken  of  by 
artists  as  "tones."  If  a  piece  of  white  paper  on  which  black 
marks  have  been  made  is  photographed  the  result  will  be 
a  picture  in  two  tones  (Fig.  53).  Between  these  extremes 
are  other  tones  spoken  of  as  halftones.  Figs.  54,  55,  and 
56  show  the  effects  of  additional  tones.    In  Fig.  57  the  six 

tones  complete  the  repre- 
sentation of  an  object,  from 
which  it  will  be  seen  that 
form  and  substance  are 
shown  by  degrees  of  bright- 
ness. In  the  mind  the 
forms  of  natural  objects 
are  comprehended  by  the 
degrees  of  brightness  that 
occur  in  them.  It  is  the 
business  of  photography  to 
reproduce  these  different 
degrees  of  brightness, which 
may  vary  from  white  to 
black. 

Differences  in  brightness 
which  occur  in  nature  may 


Fig.  55. 
Four  Tones. 


Fig.  56. 

Five  Tones. 


58 


Six  fones. 


REPRODUCTION  OF  LIGHT  AND  SHADE 


-eflects  about  10%,  and  the  '< 
m\\  reflect  about  1%  or  2%  oj 

Since  in  natural  scenes  both] 
illumination  vary,  some  parts; 
clouds  in  sunlight,  and  others 
the  range  of  contrast  is  often  v 
graphic  purposes  a  scale,  or  co 
brightest  thing  is  only  four  tii 
is  very  low,  and  such  a  subjec 
trast  of  1  to  10  is  a  medium  so 
contrast;  1  to  40  very  strong  ; 
gree  of  contrast.  All  these  d 
subjects  such  as  landscapes,  st 

Since  the  more  nearly  we  c 
the  range  of  brightnesses  whic 
ture  was  taken,  the  better  tl 
original  scene,  our  object  in  pi 
accurate  reproduction  of  the 
which  occur,  keeping  each  ton 
Jn  the  scale  as  it  occupied  in  I 
graphed.  This  is,  of  course, 
brightnesses  is  small  than  if  it 

Win^n  we  make  a  photograp 

Fig.  58. 
Front  Lighting  (Flatness). 


Fig.  59. 
Side  Lighting  (Tone  Graduations). 


be  produced  by  differences  in  the  illumination  of  the  object. 
If  a  plaster  cast  is  lighted  directly  from  the  front  the  out- 
lines will  be  visible  but  there  will  be  no  variation  in  tone.  It 
will  have  a  flat,  even  appearance  (Fig.  58).  If  the  cast  is 
lighted  from  one  side  shadows  will  be  formed,  there  will  be 
variations  in  illumination,  and  in  this  way  tones  will  be  pro- 
duced by  shadow  (Fig.  59). 

In  measuring  the  brightness  of  natural  objects,  the  eye, 
unfortunately,  cannot  be  used  directly  as  a  measuring  in- 
strument. By  lifting  a  weight  its  approximate  heaviness 
can  be  guessed  at,  but  the  eye  cannot  gauge  brightness  be- 
cause the  sensitiveness  of  the  eye  changes  according  to  the 
brightness  of  the  light.  The  eye  can,  however,  tell  very 
accurately  when  two  things  are  of  the  same  brightness,  and 
in  order  to  make  use  of  this  a  photometer  is  used.  This  is 
an  instrument  for  measuring  brightness  by  comparison  with 
a  known  brightness.  A  convenient  form  of  the  instrument 
is  shown  in  Fig.  60.    In  this  the  scene  is  viewed  through  a 

59 


FUNDAMENTALS  OF  PHOTOGRAPHY 


hole  in  a  piece  of  white  pape  is  only  possible  in  sculpture.'i 
must  be  backed  on  metal  so  1  of  brightness  and  of  color,! 
by  a  small  lamp  which  can  aftsman  uses  only  the  differ-e 
from  the  paper  is  varied.        in  the  special  branch  of  color 

In  order  to  use  the  instrur!al  onlY  with  the  reproduction  , 
that  the  brightness  to  be  me^ghtness. 
hole  in  the  paper,  and  therprightness   are   spoken   of   by 
brightness  on  the  paper  is  tpf  white  paper  on  which  black, 
the  hole,  and  then,  since  tfotographed  the  result  will  be 
throws  on  the  paper  at  cliff53).    Between  these  extremes 
shall  be  able  to  read  off  the  •  halftones.    Figs.  54,  55,  and 
is  measured.  ^nal  tones.    In  Fig.  57  the  six 

The  standard  brightness  is       tones  complete  the  repre- 


meter's  distance,  the  meter 
about  thirty-nine  inches,  an 
a  candle  meter.    In  ordinary 
about  10,000  to  30,000  candl 
will  measure,  perhaps,  1,000] 
under  a  tree,  perhaps,  100  ca 
The  brightness  of  an  obje< 
illumination  falling  upon  it, 
power  of  the  object  itself.    1 
fleeting  power.    If  a  piece  of 
fleeting  power  of  80%,  a  pi<j 

only  44%  of  the  light  falling  upon  it,  and  so  on  down  the 
scale,  a  piece  of  black  paper  reflecting  only  about  5%.  The 
brightest  thing  known  is  white  chalk,  which  reflects  90%  of 
the  light  falling  upon  it;  that  is,  of  all  the  light  falling  on 
the  white  chalk  90%  is  reflected  back.  Snow  does  not  re- 
flect quite  as  much  light  as  chalk.  The  ordinary  red  brick 
wall   reflects  only  about   20%.    Good   black   printers'   ink 


sentation  of  an  object,  from 
which  it  will  be  seen  that 
form  and  substance  are 
shown  by  degrees  of  bright- 
ness. In  the  mind  the 
forms  of  natural  objects 
are  comprehended  by  the 
degrees  of  brightness  that 
occur  in  them.  It  is  the 
business  of  photography  to 
reproduce    these    different 


nporpps  o 


f  b 


■Object 


Fig.  60. 
Photometer  to  Measure  Relative  Brightness. 

6o 


REPRODUCTION  OF  LIGHT  AND  SHADE 

reflects  about  10%,  and  the  blackest  thing,  black  velvet, 
will  reflect  about  1%  or  2%  of  the  light  falling  upon  it. 

Since  in  natural  scenes  both  the  reflecting  power  and  the 
illumination  vary,  some  parts  of  a  landscape  consisting  of 
clouds  in  sunlight,  and  others  of  dark  rocks  in  the  shade, 
the  range  of  contrast  is  often  very  considerable.  For  photo- 
graphic purposes  a  scale,  or  contrast  of  1  to  4,  in  which  the 
brightest  thing  is  only  four  times  as  bright  as  the  darkest, 
is  very  low,  and  such  a  subject  would  be  called  flat;  a  con- 
trast of  1  to  10  is  a  medium  soft  contrast;  1  to  20  a  strong 
contrast;  1  to  40  very  strong  and  1  to  100  an  extreme  de- 
gree of  contrast.  All  these  degrees  of  contrast  occur  in 
subjects  such  as  landscapes,  street  and,  seashore  scenes. 

Since  the  more  nearly  we  can  reproduce  in  our  picture 
the  range  of  brightnesses  which  were  present  when  the  pic- 
ture was  taken,  the  better  the  picture  will  represent  the 
original  scene,  our  object  in  photography  must  be  to  get  an 
accurate  reproduction  of  the  various  tones  or  brightnesses 
which  occur,  keeping  each  tone  in  its  same  relative  position 
in  the  scale  as  it  occupied  in  the  subject  which  was  photo- 
graphed. This  is,  of  course,  easier  to  do  if  the  range  of 
brightnesses  is  small  than  if  it  is  very  great. 

When  we  make  a  photograph  we  do  the  operation  in  two 
separate  steps.  We  first  make  a  negative  upon  a  highly 
sensitive  material  and  obtain  a  result  in  which  all  the  tones 
of  the  original  are  inverted,  the  brightest  part  of  the  sub- 
ject being  represented  by  a  deposit  of  silver  in  the  negative 
which  lets  through  the  least  amount  of  light,  while  the  dark- 
er parts  of  the  subject  are  represented  by  transparent  areas 
in  the  negative  which  let  through  the  most  light.  This  nega- 
tive is  then  printed  upon  a  sensitive  paper,  in  which  opera- 
tion the  scale  of  tones  is  again  reversed  so  that  the  bright 
parts  of  the  subject  which  were  represented  by  heavy  de- 
posits in  the  negative  now  appear  as  the  light  areas  of  the 
print  and  the  dark  portions  of  the  subject  which  were  trans- 
parent in  the  negative  are  represented  by  dark  deposits  in 
the  print. 

In  order  to  find  out  how  closely  the  tones  of  the  print 
follow  those  of  the  original  subject  we  must  follow  the 
changes  of  these  tones  through  both  steps:  we  must  study 
first  how  far  the  negative  reproduces  in  an  inverted  form 
the  tones  of  the  subject  and  then  how  accurately  the 
printing  paper  inverts  these  again  to  give  a  representation 
of  the  original. 

6i 


FUNDAMENTALS  OF  PHOTOGRAPHY 


B 

Fig.  61. 
Five  Toned  Block. 


Any  silver  deposit  in  the  negative  will  let  through  a  certain 
proportion  of  the  light  which  falls  upon  it.  A  very  light 
deposit  may  let  through  half  the  light,  a  dense  deposit  one- 
tenth,  a  very  dense 
deposit  one-hun- 
dredth or  even  only 
one-  thousandth. 
The  amount  of  de- 
posit through  which 
one  can  see  depends, 
of  course,  upon  the 
brightness  of  the 
scene  at  which  one 
is  looking,  but  it  is 
interesting  to  note 
that  one  can  see  the 
sun  through  a  de- 
posit which  lets 
through  only  about 
one- twenty-billionth 
of  its  light. 

These  fractions  of 
the  light  which  are 
let  through  are  re- 
ferred to  as  the  "transparency"  of  the  deposit,  and  the 
inverse  of  the  transparency  is  called  the  "opacity",  the 
opacity,  therefore, 
being  the  light-stop- 
ping power  of  the 
deposit.  A  deposit 
which  lets  through 
half  the  light,  for  in- 
stance, is  said  to 
have  a  transparency 
of  }/2  and  an  opacity 
of  2.  Similarly,  one 
which  lets  through 
one-tenthof  thelight 
has  a  transparency 
of  1/10  and  an  opac- 
ity of  10. 

If  the  negative  is 
to  be  the  exact  in- 
verse of  the  scale  ot  pjg  (,?. 
tones  of  the  subject,                 Negative  of  Five  Toned  Block. 

62 


a 

A 

a 

REPRODUCTION  OF  LIGHT  AND  SHADE 

then  the  opacities  of  the  different  areas  must  be  in  propor- 
tion to  the  brightnesses  of  the  parts  of  the  subject  which 
produce  them.  In  Fig.  61  we  have  a  subject  in  which  if  we 
take  the  black  background  as  having  a  brightness  of  1,  the 
brightest  portion  will  have  a  brightness  of  10,  and  the  other 
portion  will  be  in  proportion.  Then  when  we  make  a  nega- 
tive of  this  we  shall  get  the  picture  shown  in  Fig.  62,  and 
in  this,  if  we  measure  the  opacities  of  the  negative,  we  ought 
to  find  them  exactly  inverse  to  those  of  Fig.  61,  so  that  the 
transparency  of  the  background,  A,  would  be  ten  times  that 
of  the  table,  B,  or  the  opacity  of  the  table,  B,  will  be  ten 


Fig.  63. 
Graded  Strip  of  Exposures. 

times  that  of  the  background,  A.  Not  only  this,  but  the 
relative  opacity  of  the  deposits  in  the  areas  C,  D  and  E 
should  also  be  the  same  as  the  brightnesses  of  C.  D  and  E 
in  the  original  subject. 

It  will  be  seen  by  the  foregoing,  therefore,  that  a  tech- 
nically perfect  negative  will  be  one  in  which  the  opacities 
of  its  different  gradations  are  exactly  proportional  to  the 
light  reflected  by  those  portions  of  the  original  subject 
which  they  represent. 

Let  us  now  consider  how  far  we  can  fulfill  this  condition 
and  what  must  be  done  to  obtain  such  a  perfect  negative 
of  any  subject. 

Suppose  that  a  photographic  plate  or  film  is  exposed  to 
a  series  of  known  brightnessess;  for  instance,  that  we  photo- 
graph a  scale  made  up  of  stops  of  different  reflecting  powers 
so  the  brightness  of  each  step  is  doubled  with  regard  to  the 
next  one.  We  should  get  a  negative  which  would  look  like 
Fig.  63. 

Now  if  the  rendering  is  technically  perfect,  the  opacities 
of  this  negative  should  be  the  same  as  the  brightnesses  of 
the  different  steps  of  the  original;  that  is  to  say,  as  each 
step  is  twice  the  brightness  of  the  next  step,  the  light  let 
through  each  step  of  the  negative  should  be  half  the  amount 
of  the  step  next  to  it.  This  would  be  attained  if  each  step 
in  the  negative  added  the  same  amount  of  silver  to  the  de- 

63 


FUNDAMENTALS  OF  PHOTOGRAPHY 

posit,  so  that  if  we  could  represent  the  silver  for  each  step 
as  altering  the  thickness  of  the  silver  deposit  (it  does  not 
do  this  really,  of  course;  it  adds  to  the  number  of  grains  in 
the  same  layer)  and  then  could  cut  an  imaginary  section 
through  the  negative  so  as  to  show  the  height  of  the  deposit 


Fig.  64. 

Heights  of  Silver  Deposits 

(diagram). 


Fig.  65. 

Heights  ol  Silver  Deposits. 

(Line  Diagram). 


of  silver,  it  should  look  like  Fig.  64;  and  if  we  draw  a  dia- 
gram in  which  the  amount  of  silver  is  represented  by  the 
height  of  a  vertical  line,  the  diagram  showing  the  amount 
of  silver  for  the  different  steps  might  look  like  Fig.  65. 

If  we  actually  try  this  experiment,  however,  we  shall  find 
that  the  silver  does  not  rise  quite  uniformly  in  this  way  as 
the  exposure  is  increased  through  the  entire  scale,  but  that 
instead  we  get  the  diagram 
shown  in  Fig.  66,  and  this  dia- 
gram, which  represents  the  act- 
ual relation  between  the  silver 
deposit  in  a  photographic  ma- 
terial and  the  increase  of  ex- 
posure, requires  careful  study. 

Starting  at  A  and  proceeding 
to  B  we  notice  that  at  the  be- 
ginning, in  the  lower  exposures, 
the  steps  are  marked  by  a  grad- 
ually increasing  rise,  and,  there- 
fore, in  this  part  of  the  exposure 
scale  there  will  be  too  great  a 
gain  in  opacity  for  each  given 
increase  of  exposure.  A  nega- 
tive, the  gradations  of  which 
fall  in  this  period,  will  yield  prints  in  which  an  increasing 
contrast  is  shown  between  tones  of  uniform  increase  of 
brightness;  that  is  to  say,  it  will  appear  what  we  term 
"under-exposed."  From  this  period  at  B  we  pass  imper- 
ceptibly into  the  period  where  the  densities  show  an  equal 
rise  for  each  equal  increase  of  exposure,  and  here  we  have 

64 


?- 

^ 

, 

' 

/ 

/ 

.^r 

• 

Fig.  66. 

Diagram  <>l  Actual  Increase  and 

Graded  Strip. 


REPRODUCTION  OF  LIGHT  AND  SHADE 

our  technically  perfect  negative,  that  is,  one  in  which  the 
opacities  are  exactly  proportional  to  the  light  intensities 
of  the  subject.  This  is  termed  the  "period  of  correct  ex- 
posure," and  only  through  this  period  of  the  curve  where 
the  opacities  are  directly  proportional  to  the  exposures  and 
where  the  densities  show  an  equal  increase  each  time  the 
exposure  is  doubled  shall  we  get  a  perfect  rendering  of  the 
original  subject.  From  the  point  C  onwards  we  have  a 
gradually  decreasing  rise  in  the  steps  with  increase  of  expos- 
ure until,  finally,  the  increase  of  density  with  further  expos- 
ure becomes  imperceptible.  This  period  is  the  period  of 
"over-exposure,"  in  which  the  opacities  of  the  negative  fail 
to  respond  to  increasing  amounts  of  exposure  and  the 
correctness  of  rendering  is  again  lost.  It  will  be  seen  at 
once,  then,  from  this  curve  that  only  through  the  period 
of  correct  exposure  where  equal  increases  of  exposure  are 
represented  by  equal  rises  in  density  can  tones  of  the 
original  subject  be  correctly  reproduced  in  the  print. 

If  we  join  all  these  points  to- 
gether instead  of  representing 
them  as  a  staircase  effect,  as  is 
shown  by  dotted  line  in  Fig.  66, 
we  get  a  smooth  curve,  Fig  67, 
of  which  the  straight  line  por- 
tion (B  to  C)  represents  the 
period  of  correct  exposure, 
while  the  more  or  less  curved 
portions  at  the  beginning  and     *  B 

end  of  the  curve  correspond  to  Fig.  67. 

the  periods  of  under-exposure       Curve  Showing  Under,  Correct 

i  and  Uver-exposure. 

and  over-exposure. 

~  It  must  be  realized  that  no  ordinary  negative  can  show 
the  whole  range  of  exposures  from  beginning  to  end  of  this 
curve.  This  is  because  the  range  of  brightnesses  covered 
by  the  wrhole  curve  is  much  greater  than  that  which  occurs 
in  ordinary  subjects  and  consequently  it  is  quite  possible 
to  represent  an  ordinary  subject  entirely  in  the  period  of 
correct  exposure,  avoiding  both  the  period  of  under-expos- 
ure and  the  period  of  over-exposure.  If,  therefore,  we  wish 
to  obtain  a  technically  perfect  negative,  we  must  expose  so 
that  the  subject  which  we  are  photographing  falls  into  this 
period  of  correct  exposure,  when  we  shall  obtain  a  negative 
in  which  there  will  be  no  wholly  transparent  film,  since  this 
would  mean  that  we  had  entered  the  period  of  under-ex- 
posure, and  there  will  be  no  blocked  up  masses  of  silver 

6< 


ur?dfr  ejtpoxd 


FUNDAMENTALS  OF  PHOTOGRAPHY 


M 


since  this  would  mean  that  the  negative  was  over-exposed. 
The  capacity  of  a  photographic  material  to  render  the  scale 
of  tone  values  correctly  is,  therefore,  entirely  a  matter  of 
the  length  of  the  straight  line  portion  of  the  curve,  and  it 
is  the  length  of  this  straight  line  portion  in  the  case  of  Kodak 
film  which  gives  its  well-known  "quality"  to  the  material. 
By  the  use  of  a  material  of  this  kind  which  has  a  long 
straight  line  portion  to  the  curve,  and  of  an  exposure  which 
will  place  the  scale  of  intensities  on  that  straight  line  por- 
tion we  can  correctly  translate  the  tones  of  the  subject  into 
corresponding  opacities  in  the  negative  and  obtain  a  tech- 
nically perfect  negative. 

When  we  come  to  the  second  step  of  the  process,  however, 
and  make  a  print  from  this  negative,  we  find  that  however 
carefully  we  choose  our  exposure  and  development  perfect 
reproduction  in  the  print  is  unobtainable.  For  a  negative 
material  the  relation  between  the  silver  deposit  and  the  in- 
crease of  exposure  is  given  by  a  curve  similar  to  that  shown 
in  Fig.  67,  and  in  this  curve  the  straight  line  portion  (B  to  C) 
represents  the  period  of  correct  exposure,  so  that  to  obtain 
perfect  reproduction  in  the  negative  we  must  expose  so  that 
the  whole  range  of  brightnesses  in  the  subject  falls  within 
this  period  of  correct  exposure,  none  of  the  tones  being 
represented  by  densities  in  the  negative  which  fall  on  the 
curved  portions  at  the  beginning  and  end  of  the  curve  cor- 
responding to  the  periods  of  under  and  over-exposure. 

When  we  make  a  print,  however,  we  cannot  do  this  be- 
cause in  a  print  we  are  forced  to  use  the  whole  range  of  re- 
flecting power  of  the  printing  paper;  we  must  have  high- 
lights which  are  almost  white  paper,  and  shadows  which 
are  as  black  as  the  silver  deposit  will  give.  This  is  necessary 
because  the  total  range  of  tones  which  can  be  obtained  by 
reflected  light  is  none  too  great  for  the  reproduction  of 
natural  subjects,  while  in  negatives,  where  the  light  is  trans- 
mitted instead  of  reflected,  the  available  range  is  enormous 
and  we  need  make  use  of  only  a  small  portion  of  it.  This 
is  also  true  in  the  ease  of  transparent  positives  such  as  lan- 
tern slides  and  motion  picture  films,  which  give  the  best 
rendering  of  any  printing  material.  — — -~ 

We  can  try  the  effect  of  an  increasing  series  of  exposures 
upon  a  printing  paper  in  exactly  tin-  same  way  as  upon  a 
film,  that  is,  we  can  give  a  first  exposure  just  sufficient  to 
get  a  barely  perceptible  image  after  development,  then  ex- 
pose  another  portion  for  twice  the  time,  another  for  four 
times,   and  so  on.      Now    instead    of  measuring  the  light 

66 


REPRODUCTION  OF  LIGHT  AND  SHADE 


Fig.  68. 
Curve  of  a  Printing  Paper. 


transmitted  by  the  various  densities,  as  we  did  in  the  case 
of  the  film,  we  must  measure  the  light  reflected  from  them. 
We  get  a  series  of  "reflection  densities"  on  paper  correspond- 
to  the  transmission 
densities  of  the 
film  and  we  can  ex- 
press the  result  in 
the  form  of  a  curve 
just  as  we  did  in 
the  case  of  the  film. 
Thus  in  Fig.  68 
we  see  that  the 
densities  increase 
gradually  at  first, 
as  shown  on  the 
lower  portion  of 
the  curve,  then 
grow  in  equal  steps 
for  equal  increases 
of  exposure,  as  with 
the  film,  and  then 
the  increase  not  only  grows  less,  but  very  soon  stops  alto- 
gether, as  shown  by  the  upper  portion  of  the  curve.  This 
result  only  occurs  with  a  film  with  very  great  exposures 
indeed,  since  after  a  film  begins  to  be  over-exposed  there 
is  still  a  considerable  range  of  exposures  before  the  increase 
of  density  with  exposure  actually  ceases.  Therefore,  a  paper 
is  seen  to  differ  from  a  film  in  that  we  rapidly  reach  a  point 
where  we  have  obtained  the  maxiumm  blackness  of  deposit 
which  the  sensitive  emulsion  is  capable  of  giving  and  where 
no  further  increase  of  exposure  will  enable  us  to  obtain  a 
more  intense  black. 

The  reason  for  this  is  that  with  the  paper  we  are  dealing 
with  reflected  light,  and  not  with  transmitted  light,  as  in 
the  case  of  the  film,  and  the  light  is  reflected  from  three 
surfaces — from  the  surface  of  the  gelatine,  from  the  surface 
of  the  silver  deposit,  and  that  which  is  not  absorbed  in 
passing  through  the  silver  deposit  is  reflected  from  the  paper 
beneath. 

The  rule  for  correct  rendering  of  tones  on  the  paper  is 
the  same  as  for  the  negative;  that  is,  the  tones  which  fall  on 
the  straight  line  portion  of  the  curve  are  rendered  correctly, 
and  those  which  fall  on  the  top  and  bottom  portions  of 
the  curve  do  not  reproduce  the  tones  of  the  negative  in 
their   correct  position.     As   has   already   been    said,    how- 

6? 


FUNDAMENTALS  OF  PHOTOGRAPHY 


BLACKEST  POSSIBLE  OE, 


'S°i>T 


WHITE  PAPER 


ever,   the   difference  is  that  in  the  negative  we  can    gen- 
erally confine  the  scale  of  the  subject  to  the  straight  line 

part  of  the  curve,  while  in 
printing  we  are  forced  to  use 
the  whole  curve,  including 
those  portions  which  cannot 
give  a  perfectly  correct  ren- 
dering of  the  tones  of  the 
negative. 

Different  papers  some- 
times show  very  different 
curves;  thus  in  Fig.  69  we 
see  the  way  in  which  two 
different  papers  give  their 
scales  of  tones ;  both  give  the 
same  range  of  tones,  both 
require  the  same  range  of 
exposures  to  give  the  entire 
range  of  tones,  but  in  the 
one  the  deposit  grows  evenly  with  the  increase  of  exposure 
while  in  the  other  the  curve  is  scarcely  straight  at  all.  The 
paper  showing  the  even  growth  of  deposit  will  give  a  correct 
rendering  of  the  tones  of  the  negative  throughout  the 
greater  part  of  its  curve  (shown  by  dotted  line  in  Fig.  69) 
and  it  is  generally  said  that  such  a  paper  has  good  "quality" 
while  the  paper  with  the  uneven  growth  (solid  line  Fig.  69) 
has  poor  "quality".  For  papers,  therefore,  as  well  as  for 
negative-making  materials,  quality  depends  upon  the  pro- 
portion of  the  curve  which  is  a  straight  line,  and  the 
straighter  the  curve  the  better  the  quality. 


EXPOSURE. 

Fig.  69. 

Curves  Showing  Good  and  Poor 

"Quality"  in  a  Printing  Paper. 


68 


CHAPTER  VIII. 

PRINTING. 

AG R  EAT  number  of  different  processes  have  been  used 
at  one  time  or  another  for  printing  negatives.  The 
earliest  printing  processes  depended  upon  the  fact  that 
silver  compounds  darken  in  light,  and  the  first  printing 
paper  to  be  used  generally  was  made  by  soaking  a  sheet  of 
paper  in  a  solution  of  table  salt  and  washing  this  over  with 
a  solution  of  silver  nitrate  so  as  to  convert  the  salt  into 
silver  chloride.  Paper  so  prepared  was  known  as  "salted" 
paper  on  which,  after  exposure  to  light  behind  a  negative, 
a  print  was  obtained  which  could  be  toned  by  the  deposition 
of  gold  from  a  solution  and  then  fixed  with  hypo.  A  better 
paper  was  made  by  using  albumen  obtained  from  the  white 
of  eggs.  After  adding  salt  to  it  the  albumen  was  spread  over 
the  surface  of  the  paper  and  then  sensitized  by  treatment 
with  a  solution  of  silver  nitrate. 

After  the  gelatine  process  for  negatives  was  discovered 
gelatine  emulsions  wTere  applied  to  printing  papers.  Gela- 
tine paper  was  made  by  emulsifying  silver  chloride  in  gela- 
tine with  an  excess  of  silver  nitrate  and  then  coating  it  on 
paper  just  as  films  are  coated  with  the  sensitive  negative 
emulsion.  The  typical  gelatino-chloride  paper  of  this  type 
is  Solio. 

To  use  Solio,  the  negative  is  put  in  a  printing  frame,  and 
the  paper  is  put  with  its  coated  side  in  contact  with  the 
emulsion  side  of  the  negative  and  pressed  into  contact  by 
closing  the  back  of  the  printing  frame.  The  frame  is  then 
exposed  to  daylight  and  the  image  printed  on  the  paper, 
which  darkens  to  a  brownish-red  color.  From  time  to  time 
the  depth  of  the  printing  is  observed  by  opening  the  back 
of  the  frame.  The  image  must  be  printed  to  a  somewhat 
darker  color  than  will  be  required  in  the  finished  picture. 
When  printed  the  paper  is  removed  in  subdued  light  and 
the  print  is  toned  by  immersing  in  a  solution  containing 
gold  so  that  the  metallic  gold  is  deposited  on  the  print, 
giving  it  a  purple  color.  After  toning,  the  print  is  fixed  in 
a  hvpo  solution  and  washed.    A  toning  process  is  necessarv 

69 


FUNDAMENTALS  OF  PHOTOGRAPHY 

with  all  printing-out  silver  papers,  such  as  Solio,  albumen- 
ized  paper,  or  salted  paper,  because  if  the  printed-out  silver 
image  is  fixed  without  toning,  the  fixing  bath  changes  it  to 
an  ugly  yellow  color  and  a  very  poor-looking  print  results. 
The  gold  toning  produces  a  rich-looking,  permanent  image 
which  varies  in  color  from  brown  to  purple;  these  colors, 
indeed,  used  to  be  regarded  as  the  only  satisfactory  colors 
for  photographs. 

The  chief  use  for  printing-out  papers  at  the  present  time 
is  for  the  making  of  photographers'  proofs.  For  this  purpose 
the  negatives  are  printed,  but  the  prints  are  not  toned  or 
fixed,  and,  while  they  are  satisfactory  for  examination,  they 
cannot  be  kept,  because  they  darken  in  the  light,  the 
photographer  supplying  them  only  as  samples  to  show  the 
pose  and  expression,  and  making  permanent  prints  to  order 
later. 

Quite  early  in  the  history  of  photography  it  was  dis- 
covered that  many  substances  besides  the  salts  of  silver  are 
sensitive  to  light.  One  process  of  printing,  the  platinum 
process,  is  founded  upon  the  sensitiveness  to  light  of  iron 
salts.  If  paper  is  coated  with  ferric  oxalate,  which  is  a 
green  soluble  salt  of  iron,  and  this  is  exposed  to  light,  the 
ferric  oxalate  is  changed  into  another  oxalate  of  iron,  ferrous 
oxalate,  which  is  insoluble,  so  that  a  sheet  of  paper  thus 
prepared  and  printed  will,  after  washing,  give  a  faint  image 
consisting  of  ferrous  oxalate.  If,  to  the  ferric  oxalate  with 
which  the  paper  is  prepared,  a  solution  of  a  platinum  com- 
pound is  added  and  then,  after  printing,  the  faintly  visible 
image  is  put  into  a  solution  of  a  soluble  oxalate,  the  ferrous 
oxalate  is  dissolved  and  attacks  the  platinum  salt,  which  is 
not  affected  by  the  ferric  oxalate,  precipitating  metallic 
platinum  on  the  paper  so  that  an  image  is  obtained  con- 
sisting of  black  metallic  platinum.  Prints  made  in  this  way 
are  called  "platinum  prints"  and  since  metallic  platinum 
is  one  of  the  most  resistant  of  all  known  materials  the  pro- 
cess may  be  considered  to  give  prints  ot  the  very  greatest 
permanency. 

Another  process  depends  upon  the  fact  that  gelatine  con- 
taining bichromate  becomes  insoluble  in  water  on  exposure 
to  light,  and  this  process  is  known  as  the  "pigment"  process 
or  more  commonly  as  the  "carbon"  process,  the  name  being 
derived  from  the  fact  that  the  gelatine  used  in  the  early 
days  of  the  process  contained  finely  divided  carbon  or  lamp 
black  to  act  as  a  pigment.  The  paper  is  made  by  coating 
the  paper  stock  with  a  thick  gelatine  solution  containing 

70 


PRINTING 

finely  divided  pigment  suspended  in  it.  The  pigment  is 
chosen  according  to  the  color  of  the  print  required.  For  a 
black  image  it  may  be  lamp  black,  for  a  red  image  red  ochre 
or  burnt  sienna,  and  for  images  of  other  colors  any  perma- 
nent and  stable  pigment  of  the  color  desired  which  can  be 
finely  powdered.  After  the  coated  gelatine  has  been  dried 
the  paper  is  immersed  in  a  solution  of  bichromate  of  potash 
or  ammonia  and  again  dried.  This  bichromated  gelatine  is 
quite  soluble  in  hot  water,  but  if  it  is  exposed  to  light  it 
becomes  insoluble  where  the  light  has  acted  upon  it.  The 
bichromated  gelatine  is,  therefore,  printed  under  the  nega- 
tive in  the  same  way  as  a  Solio  print.  No  visible  image  is 
produced,  and  to  get  the  visible  print  it  is  necessary  to  wash 
away  the  soft  gelatine.  The  gelatine,  which  has  been  hard- 
ened by  the  action  of  light,  is  on  the  surface  of  the  print 
and  the  soft  gelatine  is  at  the  back,  so  in  order  to  develop 
the  print  it  is  put  face  down  on  to  another  sheet  of  paper 
and  placed  in  hot  water.  After  a  short  time  the  soluble 
gelatine  begins  to  ooze  out  at  the  edges  of  the  print  and 
the  whole  of  the  original  paper  can  be  pulled  off,  leaving  the 
image  covered  with  a  sticky  mass  of  partly  dissolved  gela- 
tine on  the  paper  to  which  it  has  been  transferred.  This 
image  is  then  washed  in  hot  water  until  all  the  soluble  gela- 
tine has  been  washed  away,  leaving  a  clear  image  of  the 
pigmented  gelatine  on  the  paper. 

All  these  printing-out  processes  which  require  a  long  ex- 
posure to  strong  daylight  however,  have  become  more  or 
less  obsolete  owing  to  the  trouble  of  working  them  and 
especially  the  difficulty  of  judging  the  correct  exposure  with 
such  a  variable  illuminant  as  daylight,  and  they  have  been 
displaced  by  printing  processes  in  which  the  paper  used  is 
coated  with  an  emulsion  very  similar  to  that  used  for  mak- 
ing the  negative,  but  of  considerably  less  sensitiveness.  This 
paper,  known  as  development  paper,  is  exposed  behind  the 
negative  to  a  lamp,  and  is  then  developed,  in  the  same  way 
as  a  negative,  to  give  a  visible  image. 

The  oldest  of  these  development  papers  is  bromide  paper. 
This  paper  is  coated  with  an  emulsion  very  similar  to  the 
ordinary  negative  emulsions  but  of  somewhat  less  sensi- 
tiveness. The  paper  is  very  sensitive  to  light  and  must  be 
worked  by  red  or  orange  light  only.  The  exposure  for  print- 
ing is,  of  course,  very  short  and  the  paper  is,  in  fact,  mostly 
used  for  enlarging,  the  image  of  the  negative  being  thrown 
upon  the  sensitive  bromide  paper  by  a  projection  lantern  so 
as  to  obtain  an  enlarged  picture  from  the  negative. 

71 


FUNDAMENTALS  OF  PHOTOGRAPHY 

About  1894  Velox  paper  was  introduced  and  was  an  en- 
tire novelty,  since  while  it  was  similar  to  bromide  paper  in 
that  it  is  exposed  to  an  artificial  light  and  then  developed 
and  fixed,  it  is  so  much  less  sensitive  than  bromide  paper 
that  it  can  be  worked  in  a  room  lighted  by  a  weak  artificial 
light  and  does  not  require  a  special  darkroom,  from  which 
fact  it  is  known  as  "gaslight"  paper.  Since  the  introduction 
of  Velox  other  gaslight  papers  have  been  made  and  at 
present  almost  all  prints  made  by  contact  from  negatives 
are  made  on  gaslight  papers,  though  Velox  is  still  the  best 
known  of  all.  Velox  is  about  a  thousand  times  slower  than 
bromide  paper  so  that  it  can  be  handled  safely  in  any  sub- 
dued light.  It  requires  an  exposure  that  ranges  from  about 
5  seconds  to  about  a  minute,  depending  on  the  density  of 
the  negative  and  the  grade  of  Velox  used,  at  one  foot  from 
a  40-watt  mazda  lamp,  and  it  is  characterized  especially  by 
the  extreme  rapidity  and  ease  of  its  development,  from 
which  its  name  is  derived,  Contrast  and  Regular  developing 
fully  in  15  to  20  seconds  and  Special  Velox  in  about  30  sec- 
onds. It  is  consequently  possible  by  using  Velox  to  make 
prints  in  comfort  and  with  great  rapidity,  the  old  troubles 
of  judging  the  extent  of  the  printing,  and  the  difficulties 
with  toning  baths  being  entirely  absent  with  this  simple 
and  convenient  printing  medium. 


Fig.  70. 
Degrees  of  Light  Intensities. 

Velox  paper  is  made  in  three  grades  of  contrast  to  til  dif- 
ferent types  of  negatives.  The  paper  was  originally  made 
in  the  Regular  grade  only,  but  it  was  found  that  many 
negatives  were  too  contrasty  to  print  well  on  this  paper  and 
Special  Yelox  was  manufactured  for  use  with  such  negatives, 
while  recently  Contrast  Velox  has  been  put  on  the  market 
for  use  with  negatives  so  lacking  in  contrast  that  they  will 
not  give  good  prints  even  on  the  Regular  grade. 

If  we  make  three  negatives  of  the  same  subjeel  in  succes- 
sion, giving  each  exactly  the  same  exposure,  and  then  dev- 
elop these  for  different  lengths  of  time  so  th.it  the  first  will 
be  underdeveloped,  the  second  correctly  developed  and  the 

72 


PRINTING 


51 

riN 

* 

pr  ^ 

Soft  Negative  of  Little  Contrast. 


Print  from  Opposite  Negative 
on  Contrast  Yelox. 


Average  Negative  of  Medium  Contrast. 


Print  from  Opposite  Negative 
on  Regular  Yelox. 


Hard  Negative  of  Strong  Contrast. 


Fig.  71. 


Print  from  Opposite  Negative 
on  Special  Yelox. 


third  OYer  developed,  the  first  negative  will  have  a  short 
range  of  contrast,  the  second  a  medium  range,  and  the  third 
a  long  range.  If  we  then  print  the  first  negative  on  Contrast 
Velox,  the  second  on  Regular  Velox,  and  the  third  on  Special 
Velox,  we  shall  get  almost  identical  prints  on  all  three  papers 
provided  that  the  contrasts  of  the  negatives  just  fit  the 
various  grades  of  the  paper.    This  is  shown  in  Fig.  71. 

73 


FUNDAMENTALS  OF  PHOTOGRAPHY 

We  might  think  that  Contrast  Velox  would  always  give 
more  contrasty  prints  than  Regular  Velox;  it  will  if  both 
papers  are  printed  on  the  same  negative,  but  if  the  Contrast 
Velox  is  printed  on  a  flat  negative  and  the  Regular  Velox 
on  a  normal  negative,  then  the  Contrast  Velox  will  compen- 


Fig.  72.     Range  of  44  Distinct  Tones. 

sate  for  the  flat  negative  and  give  a  normal  print,  just  as  the 
Regular  Velox  gives  a  normal  print  from  a  normal  negative, 
and  the  Special  Velox  a  normal  print  from  a  contrasty 
negative. 

All  the  grades  of  Velox  give  the  same  range  of  reflecting 
powers  in  the  print  provided  that  they  are  used  with  nega- 
tives which  will  enable  this  range  to  develop.  Suppose  we 
take  a  black  wedge  which  contains  all  the  degrees  of  light 
intensities,  from  absolute  opacity  at  one  end  to  absolute- 
transparency  at  the  other  end  and  make  a  print  of  it.  We 
should  get  the  result  shown  in  Fig.  70.  This  shows  the 
entire    range    of    reflecting  power  of  which    the   paper  is 

capable,  the  range  varying 
from  white  paper  at  one 
vm\  to  the  blackest  silver 
deposit  which  the  paper 
can  give,  at  the  other. 

With  any  "velvet"  sur- 
face paper,  such  as  Velvet 
Velox,  we  shall  find  that 
the  white  paper  will  refleel 
about  twenty-five  times  as 
much  light  as  the  deepest 
silver  deposit.  The  number 
of  distinct  tones  which  are 
included  in  this  range  from 
white  to  black  depends,  ol 
course,  on  the  ability  of  the 
eye  to  distinguish  them. 
The  eye  can  actually  see 
about  one  hundred  distinct 
tones  in  such  a  range. 

74 


Velox 


PRINTING 


Fig.  74. 
Print  Showing  Empty  Highlights. 


m  Fig.  75. 
Print  Showing  Blocked  Shadows. 

75 


FUNDAMENTALS  OF  PHOTOGRAPHY 

In  Fig.  72  is  shown  a  range  of  tones  made  up,  not  as  a 
continuous  wedge,  but  of  forty-four  distinct  tones.  The 
number  which  can  be  seen  in  the  illustration  is  less  than 
the  number  which  the  eye  can  distinguish  in  a  print  because 
of  the  limitations  imposed  by  the  process  of  half-tone  re- 
production. If  the  full  one  hundred  tones  which  the  eye 
can  distinguish  in  a  print  were  reproduced  by  the  half-tone 

process  the  half- 
tone illustration 
would  look  like  a 
continuous  wedge. 
In  Fig.  73  the 
same  wedge  has 
been  printed  on 
all  three  papers, 
and  it  will  be  seen 
that  Contrast  Ve- 
lox  has  reached 
its  full  blackness 
only  a  short  dis- 
tance  up  the 
wedge,  Regular 
Velox  has  gone 
farther,  and  Spec- 
ial Velox  has  gone 
the  farthest  of  all, 
so  that  while  all 
three  papers  will 
give  the  same 
range  of  tones,  this  range  is  impressed  on  Contrast  Velox 
with  only  a  short  range  of  densities  in  the  negative;  for 
Regular  Velox  a  longer  range  is  needed,  and  for  Special 
Velox  a  still  longer  range. 

The. range  of  densities  required  in  a  negative  to  just  print 
out  the  full  range  of  tones  on  a  paper  is  called  the  "scale"  of 
the  paper  and  this  is  measured  by  trying  an  increasing 
series  of  exposures  until  the  range  of  exposures  which  will 
just  give  the  whole  range  of  tones  on  the  paper  is  found; 
that  is,  if  an  exposure  of  one  second  to  the  bare  paper  with 
no  negative  will  just  give  the  first  perceptible  difference 
from  white  paper,  so  as  to  show  the  first  trace  of  tint  on  the 
paper,  and  an  exposure  of  twenty  seconds  will  give  the 
deepest  black  the  paper  is  capable  of  rendering,  so  that  no 
increase  of  exposure  will  produce  any  denser  black,  then  we 
should  call  the  scale  of  the  printing  paper  1  to  20. 

76 


Fig.  76. 
Gray,  Flat  Print. 


PRINTING 

Thus  the  word  "scale"  applied  to  a  printing  paper  does 
not  refer  at  all  to  the  range  of  tones  in  the  print.  It  indi- 
cates the  range  of  contrast  in  the  negative  which  should 
be  printed  on  that  paper.   A  paper  with  a  scale  of  1  to  20 


Fig.  77. 
Growth  of  Contrast  with  Devel- 
opment, Eastman  N.  C.  Film. 


Fig.  78. 
Increase  of  Density  with  Devel- 
opment, Velox  Paper. 


will  require  a  negative  in  which  the  densest  part  lets  through 
1/20  of  the  light  transmitted  by  the  clearest  part,  because 
if  this  negative  is  printed  on  that  paper  the  print  will  just 
have  the  whole  range  of  tones  from  white  to  black  complete- 
ly printed  out,  each  tone  in  the  print  corresponding  to  a 
density  in  the  negative,  and  there  will  be  no  differences  of 
density  in  the  negative  unrepresented  by  differences  of  tone 
in  the  print. 

Special  Velox  has  a  scale  of  about  1  to  20  and  is  suitable 
for  printing  from  contrasty  negatives.  Regular  Velox  has 
a  scale  of  about  1  to  10  and  is  suitable  for  printing  from 
negatives  of  moderate  contrast,  while  the  very  flattest  and 
least  contrasty  negatives,  which  are  the  result  either  of  ex- 
cessive over-exposure  or  underdevelopment  should  be  print- 
ed on  Contrast  Velox,  which  has  a  scale  of  about  1  to  5. 

It  is  important  to  choose'  the  grade  of  paper  correctly  for 
the  negative.  If  the  paper  is  too  contrasty  for  the  negative; 
if,  for  instance,  we  print  a  hard  negative  (one  that  has 
strong  contrast)  on  Contrast  Velox,  then  we  shall  have  to 
sacrifice  a  part  of  the  scale  of  the  negative;  either  we  shall 
get  the  highlights  empty  and  white,  as  shown  in  Fig.  74,  or 
we  shall  get  the  shadows  blocked  up,  as  shown  in  Fig.  75. 
On  the  other  hand,  if  the  scale  of  the  paper  is  too  long  for 

77 


FUNDAMENTALS  OF  PHOTOGRAPHY 

the  negative  and  we  print  a  soft  negative  (one  that  has 
little  contrast)  on  Special  Velox,  for  instance,  when  we 
should  have  used  Regular  Velox,  then  we  shall  get  a  gray, 
flat  print,  as  is  shown  in  Fig.  76. 

With  paper,  as  with  film,  the  density  of  the  picture  is 
controlled  by  the  duration  of  the  exposure  and  the  develop- 
ment, but  whereas  with  films  the  contrast  is  dependent  upon 
the  time  of  development,  the  contrast  increasing  as  the 
development  is  continued,  with  paper  the  contrast  is  fixed 
by  the  maker,  and  after  a  few  seconds  the  development 
does  not  change  the  contrast  of  the  print  at  all  but  only 
affects  the  density  of  the  deposit.  This  is  illustrated  in 
Figs.  77  and  78. 

In  Fig.  77  we  see  that  with  increasing  time  of  develop- 
ment, a  film  shows  an  increase  in  contrast,  while  in  Fig.  78 
that  by  prolonging  development  it  is  clear  after  reaching  a 
certain  stage  in  the  development  of  a  print  there  is  only 
an  increase  in  total  density  and  no  increase  in  contrast. 

If  a  print  is  over-exposed,  it  can  be  taken  out  of  the  dev- 
eloper before  it  is  fully  developed,  and  if  under-exposed,  it 
can  similarly  be  forced  in  development,  though  there  is 
some  risk  of  yellow  stain  if  development  is  continued  too 
long.  The  best  results  can,  of  course,  only  be  obtained  by 
getting  the  exposure  right  and  giving  the  normal  time  of 
development,  which  is  from  15  to  20  seconds  for  Contrast 
and  Regular  Velox,  and  about  30  seconds  for  Special  Velox. 
The  matter  of  greatest  importance  for  getting  really  first- 
class  prints,  therefore,  is  to  give  them  the  right  time  of 
exposure. 

Before  starting  to  print  a  number  of  negatives  they 
should  be  classified  for  contrast  so  as  to  choose  a  suitable 
grade  of  paper  for  printing  them;  that  is  to  say,  put  the 
negatives  in  three  envelopes  according  to  whether  they  are 
to  be  printed  on  Special,  Regular  or  Contrast  Velox.  Now 
take  the  negatives  in  each  of  these  envelopes  and  divide 
them  again  into  three  more  classes — normal  negatives  hav- 
ing average  density,  thin  negatives,  and  dense  negatives. 
When  printing,  if  we  take  the  exposure  for  the  normal 
negative  as  standard,  then  the  thin  negatives  will  require 
half  this  standard  exposure  and  the  dense  negatives  will 
require  twice,  while  sometimes  we  may  possibly  meet  an 
exceptional  negative — very  thin  or  very  dense — which  may 
require  one-fourth  or  four  times  the  standard  exposure. 
Having  classified  our  negatives  in  this  way,  in  order  to  get 
our  exposures  right  we  need  know  onlv  the  exposure  on  each 

78 


PRINTING 

grade  of  Velox  paper  for  our  standard  negatives,  and  if  we 
print  with  a  25-watt  tungsten  lamp  at  a  distance  of  one  foot, 
we  shall  find  that  the  exposure  for  a  standard  negative 
will  be  about  20  seconds  for  Special  Velox,  about  one 
minute  for  Regular,  and  one  and  a  half  minutes  for  Contrast. 
These  figures  are  to  be  taken  only  as  a  guide,  and  when  a 
new  light  or  a  new  package  of  paper  is  used  for  the  first 
time,  trial  exposures  should  be  made  with  the  standard 
negative,  giving,  say,  15,  20  and  30  seconds  exposure,  so 
as  to  select  the  exposure  which  develops  to  the  right  density 
with  the  correct  time  of  development. 

It  is  best  always  to  use  the  same  standard  negative  for 
testing  a  new  paper  or  a  new  printing  lamp  and  any  other 
new  conditions  that  may  arise  in  printing,  as  more  useful 
information  will  be  gained  by  making  tests  with  one  nega- 
tive only  than  if  a  different  negative  is  selected  each  time 
a  test  is  to  be  made. 

If  the  subject  of  exposure  is  dealt  with  in  this  way,  if  the 
negatives  are  classified  for  density  before  printing,  and  a 
test  is  made  on  a  standard  negative,  it  will  be  found  easy 
to  print  a  large  number  of  negatives  on  several  grades  of 
Velox  paper  and  get  a  very  high  percentage  of  first-class 
prints  with  normal  development. 

With  regard  to  development  and  after-treatment  of  the 
print,  there  is  very  little  to  say,  since  the  matter  is  fully  ex- 
plained in  the  instruction  sheet  that  accompanies  each  pack- 
age of  Velox  paper.  It  is  best  to  buy  the  ready  prepared 
developers  such  as  Velox  Liquid  Developer  or  Nepera  Solu- 
tion and  to  follow  the  directions  given. 

When  fixing  prints,  take  care  that  they  do  not  lie  on  top 
of  one  another  in  the  fixing  bath  and  see  that  each  print 
gets  its  supply  of  fresh  acid  hypo. 

While  contact  prints  are  satisfactory  to  show  one's 
friends,  a  time  comes  when  we  want  to  attempt  something 
more  ambitious  and  to  make  photographs  which  we  can 
hang  on  our  walls  or  submit  for  exhibition,  and  then  we  feel 
that  we  want  something  more  than  an  ordinary  print  and 
something  more  than  an  enlarged  print;  we  want  to  make 
a  picture.  The  difference  between  a  picture  and  a  print  is 
of  course,  not  a  matter  of  size;  it  is  a  matter  of  composition 
and  balance,  of  judgment  in  the  choice  of  subject  and  of 
the  moment  of  exposure,  and  of  finish  and  quality  in  the 
result. 

The  possibility  of  using  a  very  great  degree  of  enlarge- 
ment is  shown  in  Fig.  79,  where  the  small  image  in  the  cor- 

79 


FUNDAMENTALS  OF  PHOTOGRAPHY 


.fctS«Bt 


IS 


Fig.  79. 

Extreme  Enlargement. 

Original  in  lower  right  hand  corner. 

ner  represents  a  contact  print  from  the  original  negative. 
In  this  case  the  negative  was  a  portion  of  a  motion  picture 
film  which  was  taken  to  get  the  utmost  sharpness  of  defini- 
tion and  was  then  enlarged  to  about  a  thousand  times  its 
original  size,  the  definition  in  the  finished  enlargement 
being  still  quite  good.  Such  work  as  this  is  rarely  wanted, 
but  the  great  value  of  enlarging  is  that  parts  can  be  chosen 
from  a  negative  and  enlarged  to  make  very  pleasing  pic- 
tures, where  the  whole  negative  if  printed  as  a  contact  print 
would  be  by  no  means  satisfactory.  The  print  shown  in 
Fig.  80,  for  instance,  is  an  enlargement  of  a  film  negative! 
This  negative  was  taken  at  the  seashore  as  a  snapshot  ex- 
posure, the  figures  being  very  small  and  in  the  corner  of 
the  negative  so  that  if  the  negative  were  printed  as  a  whole 
it  would  be  very  unsatisfactory.  While  a  contact  print 
trimmed  as  is  shown  in  the  enlargement  was  not  much 
larger  than  a  postage  stamp,  an  enlargement  of  the  figures 
in  it,  however,  made  a  pleasing  picture. 

Another  illustration  of  what  can  be  done  in  enlarging  is 
shown  in  Fig.  81,  where  two  negatives  have  been  enlarged 
together  to  make  a  combined  picture.    The  lower  half  of 

8o 


PRINTING 


the  original  scene,  of  which  the  church  and  trees  form  the 
upper  half,  consisted  of  a  plowed  field,  so  that  the  fore- 
ground in  the  original  negative  was  very  unsatisfactory. 
By  taking  another 
foreground,  however, 
taking  care,  of  course, 
that  the  lighting  was 
the  same,  and  shading 
the  foreground  of  the 
first  negative  so  that 
it  did  not  print  in  en- 
larging, then  chang- 
ing the  negative  in 
enlarging  and  substi- 
tuting the  foreground 
negative,  the  two  have 
been  printed  into  one 
another  with  the  re- 
sult shown.  Some 
photographers  are 
very  clever  at  making 
these  combined  en- 
largements. 

There  are  two  prac- 
tical methods  of  mak- 
ing enlargements; 
those  involving  work- 
ing in  a  dark-room, 
and  those  in  which  no 
dark-room  is  employ- 
ed for  the  enlarging 
itself.     For  the  latter 

purpose  the  Brownie  enlarging  camera  is  suitable,  this  be- 
ing simply  a  cone-shaped  box  with  a  holder  for  the  paper 
at  the  large  end  and  a  negative  holder  at  the  small  end. 
The  lens  is  fitted  inside  the  cone,  at  just  the  right  distance 
to  insure  a  sharp  focus  so  that  the  camera  is  always  focused, 
and  sharp  enlargements  are  certain  if  the  negatives  are 
sharp.  This  enlarger  is  exposed  to  daylight.  The  disadvan- 
tage with  this  camera  is  that  the  degree  of  enlargement  is 
fixed  and  that  consequently  it  is  not  easy  to  select  a  small 
portion  of  a  negative  and  enlarge  it  to  a  considerable  extent. 

Another  good  arrangement  is  that  shown  in  Fig.  82, 
where  the  film  or  glass  negative  is  put  into  the  negative 
holder  of  the  enlarging  outfit.    With  this  arrangement  the 

81 


Fig.  80. 
Enlargement,  of  Part  of  a  Snapshot. 


FUNDAMENTALS  OF  PHOTOGRAPHY 

negative  is  projected  on  to  an  easel  or  wall  on  which  the 
bromide  paper  can  be  pinned,  and  since  the  distance  of  the 
enlarger  from  the  easel  or  wall  can  be  regulated,  any  degree 
of  enlargement  can  be  obtained  and  a  small  part  of  the 
negative  can  be  selected  and  enlarged  to  any  required  size. 
In  the  earlier  printing  processes  used  by  photographers 
— those  in  which  the  image  was  obtained  by  the  continued 
action  of  light  and  which  were  toned  by  the  deposition  of 
gold  from  a  toning  bath — the  prints  obtained  were  in  var- 
ious shades  of  purple  and  brown,  and  these  shades  became 
so  associated  with  photographs  in  the  minds  of  the  public 
that  when  the  black  and  white  prints  made  on  Velox  and 
bromide  papers  began  to  displace  the  earlier  Solio  and 
Aristotype  prints,  the  general  public  would  scarcely  recog- 
nize them  as  "photo- 
graphs" at  all,  and  a 
demand  soon  arose 
for  some  method  of 
toning  the  black  im- 
ages of  bromide  and 
Velox  prints  to  a 
brown  or  sepia  simi- 
lar to  that  of  the  gold 
toned  printed -out 
papers. 

1 1  seemsto  be  char- 
acteristic of  mankind 
i  to  want  what  they 
have  not  got,  and  it, 
is  inreresung  to  note 
that  with  the  earlier 
printing-out  processes 
which  easily  gave 
warm  tones,  chemists 
were  anxiously  work- 
ing to  get  methods  of 
obtaining  black  and 
white  prints,  while 
with  the  developing- 
out   processes,   which 


■  •&  1 

■  vt  i 

VvW      > 

<3> 

rt  •iKi|  JBS 

\jftL  ^-^ 

5Jjf| 

fc'T. 

■    ■$k,aj)BB»Kli 

FjL 

WS 

^^^^PJPt 

N 


Fig.  81. 

Combined  Enlargement  from 

Two  Negatives. 


naturally  give  good  black  and  white  prints,  photographers 
desire  to  obtain  warm  sepia  and  brown  tones. 

The  processes  for  obtaining  sepia  prints  from  the  black 
developed-out  images  all  depend  on  one  chemical  reaction; 
namely,   that  by  which  silver  bromide  is  converted  into 

82 


PRINTING 

silver  sulphide.  Silver  sulphide  is  a  dark  colored,  almost 
black,  substance  well  known  to  the  housekeeper — if  not  by 
name —  as  the  tarnish  which  appears  on  silverware  after  it 
has  been  some  time  in  the  air,  the  surface  of  metallic  silver 
being  attacked  by  sulphur  compounds  in  the  air,  which 
generally  come  from  the  products  of  combustion  of  gas  in 
the  cooking  range. 

Now,  when  any  chemical  substances  can  be  produced  by 
the  interaction  of  two  other  chemical  substances  in  solution 


Fig.  82. 
Kodak  Enlarging  Outfit. 

the  question  as  to  whether  it  will  be  produced  depends  upon 
whether  it  is  more  or  less  soluble  than  the  substances  which 
can  form  it.  Silver  sulphide  is  less  soluble  than  silver  bro- 
mide so  that  when  silver  bromide  is  treated  with  a  solution 
containing  sulphur  in  a  free  form  it  is  changed  into  silver 
sulphide  and  the  silver  sulphide  is  deposited  in  its  place. 
On  the  other  hand,  metallic  silver,  such  as  that  which  forms 
the  image  in  a  developed  print,  is  less  soluble  than  silver 
sulphide  and  consequently  we  cannot  change  it  into  silver 
sulphide  by  simply  treating  it  with  a  solution  containing 
free  sulphur,  but  if  in  this  solution  we  have  some  substance 
which  will  dissolve  metallic  silver,  then  we  can  change  the 
metallic  silver  itself  into  silver  sulphide.  It  is  on  these  prin- 
ciples that  the  sulphur  toning  processes  are  based. 

One  toning  process  depends  upon  changing  the  silver 
image  of  the  print  back  into  silver  bromide.  Now,  we  know 
that  silver  is  obtained  from  silver  bromide  by  reduction, 
just  as  iron  is  got  out  of  iron  ore,  and  therefore  we  can  get 
back  silver  bromide  from  silver  by  oxidation,  which  is  the 
reverse  process  to  reduction.  If  we  use  any  solution  which 
will  oxidize  silver  and  have  potassium  bromide  present  in  the 
solution,  the  silver  image  will  be  turned  into  silver  bromide. 

83 


FUNDAMENTALS  OF  PHOTOGRAPHY 

The  usual  way  to  do  this  is  to  treat  the  black  print  after 
fixing  and  washing  with  a  solution  containing  potassium 
ferricyanide,  which  is  an  oxidizing  agent,  and  potassium 
bromide,  and  this  turns  the  black  silver  image  into  a  yellow- 
ish-white image  of  silver  bromide  which  is  scarcely  visible, 
so  that  the  process  is  called  "bleaching"  since  the  black 
silver  turns  into  white  silver  bromide,  and  then  after  wash- 
ing, this  silver  bromide  is  treated  with  a  solution  of  sodium 
sulphide,  which  turns  it  into  the  brown  silver  sulphide 
which  gives  us  our  sepia  toned  print.  So,  to  make  a  sepia 
Velox  print  by  this  method,  we  treat  it  with  the  "bleaching 
solution,"  which  turns  the  silver  into  silver  bromide,  and 
then  "redevelop"  this,  as  it  is  called,  in  a  solution  of  sul- 
phide, which  converts  the  silver  bromide  into  silver  sulphide 
and  gives  us  our  sepia  print. 

There  is  another  method  of  obtaining  sulphide  toned 
prints  which  is  somewhat  simpler.  We  have  seen  that  we 
cannot  turn  silver  directly  into  silver  sulphide  by  a  solution 
containing  free  sulphur  unless  we  have  a  solvent  of  silver 
present  in  the  solution.  Now,  it  so  happens  that  hypo  is  to 
some  extent  a  solvent  of  silver,  and  also  that  with  a  weak 
acid,  hypo  gives  free  sulphur.  Alum  is  a  weak  acid  and  it 
also  has  the  valuable  property  of  hardening  the  print,  so  if 
we  put  the  print  which  we  wish  to  tone  into  a  solution  con- 
taining hypo  and  alum,  the  silver  will  slowly  be  changed 
into  silver  sulphide  and  the  print  will  be  toned  brown.  This 
change  goes  on  very  slowly  at  ordinary  temperatures,  but 
by  heating  the  solution  it  goes  much  more  rapidly,  so  thai 
if  we  heat  a  bromide  or  Velox  print  in  a  solution  containing 
hypo  and  alum,  we  shall  get  a  good  sepia  tone  at  the  end  of 
ten  or  twenty  minutes  without  any  further  difficulty,  the 
only  objection  being  that  the  bath,  like  all  baths  containing 
free  sulphur,  and  like  the  sodium  sulphide  used  for  redevel- 
oping in  the  other  toning  process,  smells  rather  unpleasantly. 

Equally  good  results  in  sepia  toning  cannot  be  got  with 
all  papers,  but  a  great  deal  depends  on  the  development  of 
the  print.  To  get  good  sepias,  development  should  be  full; 
an  underdeveloped  print  will  always  give  weak,  yellowish 
tones  when  compared  with  one  in  which  development  has 
been  carried  out  thoroughly,  which  will  give  a  strong,  pure 
sepia.  It  is  important  to  remember  this,  as  two  prints 
which  may  look  alike  as  black  and  white  prints  will  tone  dif- 
ferently if  they  have  not  been  developed  to  the  same  extent. 


84 


CHAPTER  IX. 

THE  FINISHING  OF  THE  NEGATIVE. 

AFTER  development,  the  undeveloped  silver  bromide  is 
removed  by  immersion  of  the  negative  or  print  in  what 
is  called  the  "fixing  bath".  There  are  only  a  few  substances 
which  will  dissolve  silver  bromide,  and  the  one  which  is 
universally  used  in  modern  photography  is  sodium  thio- 
sulphate,  which  is  known  to  photographers  as  hyposulphite 
of  soda,  or  more  usually  as  hypo,  though  the  name  hypo- 
sulphite of  soda  is  used  by  chemists  for  another  substance. 

In  the  process  of  fixation  the  silver  bromide  is  dissolved 
in  the  hypo  by  combining  with  it  to  form  a  compound 
sodium  silver  thiosulphate.  Two  of  these  compound  thio- 
sulphates  exist,  one  of  them  being  almost  insoluble  in  water, 
while  the  other  is  very  soluble.  As  long  as  the  fixing  bath 
has  any  appreciable  fixing  power,  the  soluble  compound  only 
is  formed. 

Fixing  is  accomplished  by  means  of  hypo  only,  but 
materials  are  usually  transferred  from  the  developer  to  the 
fixing  bath  with  very  little  rinsing  so  that  a  good  deal  of 
developer  is  carried  over  into  the  fixing  bath,  and  this  soon 
oxidizes  in  the  bath,  turning  it  brown,  and  staining  nega- 
tives or  prints.  In  order  to  avoid  this  the  bath  has  sulphite 
of  soda  added  to  it  as  a  preservative  against  oxidation,  and 
the  preservative  action  is,  of  course,  greater  if  the  bath  is 
kept  in  a  slightly  acid  state.  In  order  to  prevent  the  gela- 
tine from  swelling  and  softening  it  is  also  usual  to  add  some 
hardening  agent  to  the  fixing  bath  so  that  a  fixing  bath  in- 
stead of  containing  only  hypo  will  contain  in  addition  sul- 
phite, acid  and  hardener. 

Now,  if  a  few  drops  of  acid,  such  as  sulphuric  or  hydro- 
chloric acid,  are  added  to  a  weak  solution  of  hypo,  the  hypo 
will  be  decomposed  and  the  solution  will  become  milky, 
owing  to  the  precipitation  of  sulphur.  The  change  of  thio- 
sulphate into  sulphite  and  sulphur  is  reversible,  since,  if  we 
boil  together  sulphite  and  sulphur  we  shall  get  thiosulphate 
formed,  so  that  while  acids  free  sulphur  from  the  hypo, 

85 


FUNDAMENTALS  OF  PHOTOGRAPHY 

sulphite  combines  with  the  sulphur  to  form  hypo  again. 
Consequently,  we  can  prevent  acid  decomposing  the  hypo 
if  we  have  enough  sulphite  present,  since  the  sulphite  works 
in  the  opposite  direction  to  the  acid.  An  acid  fixing  bath, 
therefore,  is  preserved  from  decomposition  by  the  sulphite, 
which  also  serves  to  prevent  the  oxidation  of  developer 
carried  over  into  it. 

Since  in  fixing  baths  what  we  require  is  a  large  amount 
of  a  weak  acid,  the  best  acid  for  the  purpose  is  acetic  acid. 
Citric  or  tartaric  acids  can  also  be  used. 

In  order  to  make  sure  that  the  films  are  properly  fixed 
they  should  be  left  in  the  fixing  bath  twice  as  long  as  is 
necessary  to  clear  them  from  the  visible,  white  silver  bro- 
mide. If  considerable  work  is  being  done,  the  best  course  is 
to  use  two  fixing  baths,  transferring  the  films  or  prints  to 
the  second  clean  bath  after  they  have  been  fixed  in  the  first. 
Then,  when  the  first  bath  begins  to  work  slowly,  it  can  be 
discarded  and  replaced  by  the  second  bath,  a  fresh  solution 
being  used  for  the  second  bath.  These  precautions  are 
necessary  because,  as  has  already  been  said,  silver  forms 
two  compound  thiosulphates,  the  first  of  which  is  almost 
insoluble  in  water  but  is  transformed  into  the  second,  which 
is  soluble,  by  longer  treatment  with  hypo.  Consequently 
when  a  film  first  clears,  it  still  contains  the  first  insoluble 
thiosulphate  of  silver,  and  if  it  is  taken  out  of  the  fixing 
bath  and  washed  some  of  the  silver  will  be  left  behind  and 
not  washed  out.  Then,  on  keeping,  this  silver  thiosulphate 
left  in  the  negative  will  decompose  and  produce  stains.  If 
a  negative  or  print  is  properly  fixed  and  washed  it  will  be 
permanent. 

The  actual  rate  of  washing  may  be  understood  by  remem- 
bering that  the  amount  of  hypo  remaining  in  the  gelatine 
is  continually  halved  in  the  same  period  of  time  as  the  wash- 
ing proceeds.  An  average  negative,  for  instance,  will  give 
up  half  its  hypo  in  two  minutes,  so  that  at  the  end  of  two 
minutes  half  the  hypo  will  be  remaining  in  it,  after  four 
minutes  one-quarter,  after  six  minutes  one-eighth,  after 
eight  minutes  one-sixteenth,  ten  minutes  one-thirty-second, 
and  so  on.  It  will  be  seen  that  in  a  short  time  the  amount 
of  hypo  remaining  will  be  infinitesimal.  This,  however,  as- 
sumes that  the  negative  is  continually  exposed  to  fresh 
water,  which  is  the  most  important  matter  in  arranging 
the  washing  of  either  negatives  or  prints. 

If  a  lot  of  prints  are  put  in  a  tray  and  water  allowed  to 
splash  on  the  top  of  the  tray,  it  is  very  easy  for  the  water  on 

86 


THE  FINISHING  OF  THE  NEGATIVE 

the  top  to  run  off  again,  and  for  the  prints  at  the  bottom  to 
lie  soaking  in  a  pool  of  fairly  strong  hypo  solution,  which 
is  much  heavier  than  water  and  which  will  fall  to  the  bottom 
of  the  tray.  If  the  object  is  to  get  the  quickest  washing, 
washing  tanks  should  be  arranged  so  that  the  water  is  con- 
tinuously and  completely  changed  and  the  prints  or  nega- 
tives are  subjected  to  a  continuous  current  of  fresh  water. 
If  water  is  of  value,  and  it  is  desired  to  economize  in  its  use, 
then  by  far  the  most  effective  way  of  washing  is  to  use  suc- 
cessive changes  of  small  quantities  of  water,  putting  the 
prints  first  in  one  tray,  leaving  them  there  for  from  two  min- 
utes to  five  minutes,  and  then  transferring  them  to  an  en- 
tirely fresh  lot  of  water,  repeating  this  until  they  are  washed. 

The  progress  of  the  washing  can  be  followed  by  adding 
a  little  permanganate  solution  to  the  wash  water  after  the 
prints  are  taken  out  of  it  in  order  to  see  how  much  hypo  is 
left  in  it,  the  presence  of  hypo  being  seen  by  decoloration 
of  the  permanganate.  An  even  simpler  test  is  to  taste  the 
prints.  Six  changes  of  five  minutes  each  should  be  sufficient 
to  eliminate  the  hypo  effectively  from  any  ordinary  material. 

REDUCTION. 

Sometimes  negatives  are  obtained  which  are  so  dense  that 
they  are  difficult  to  print.  Other  negatives  are  so  contrasty 
that  they  give  harsh  prints.  In  order  to  improve  these 
negatives  recourse  may  be  had  to  the  process  called  "re- 
duction," that  is,  to  the  removal  of  some  of  the  silver  by 
treatment  with  a  chemical  which  dissolves  the  metallic 
silver  of  the  image. 

It  is  unfortunate  that  the  word  "reduction"  is  used  in 
English  for  this  purpose.  In  other  languages  the  word 
"weakening"  is  used  and  it  is  undoubtedly  a  better  word 
because  the  chemical  action  involved  in  the  removal  of 
silver  from  a  negative  is  oxidation,  and  the  use  of  the  word 
reduction  leads  to  confusion  with  true  chemical  reduction 
such  as  occurs  in  development. 

In  order  to  produce  the  best  results  it  is  necessary  that 
the  reduction  should  be  suitable  for  the  negative  which  is 
to  be  treated.  Thus,  in  the  case  of  a  negative  which  is  too 
dense  all  over  it  is  necessary  to  remove  the  density  uni- 
formly, while  in  the  case  of  one  which  is  too  contrasty  what 
is  required  is  not  the  removal  of  the  silver  from  highlights 
and  shadows  alike,  but  the  lessening  of  the  deposit  on  the 
highlights  without  affecting  the  shadows. 

87 


FUNDAMENTALS  OF  PHOTOGRAPHY 

In  Fig.  83  we  see  a  diagram  which  represents  a  negative 
originally  dense  from  which  by  the  removal  of  an  equal 
amount  of  silver  from  shadows,  halftones  and  highlights, 


Fig.  83. 
Diagram  Showing  How  Cutting  Reducer  Acts. 


there  can  be  obtained  a  negative  of  proper  gradation.  A 
reducer  which  effects  this  uniform  removal  of  density  is 
generally  called  a  "cutting  reducer".  The  typical  "cutting 
reducer"  is  that  known  as  Farmer's  reducer,  which  is  made 
by  preparing  a  strong  solution  of  potassium  ferricyanide, 
otherwise  known  as  Red  Prussiate  of  Potash,  and  adding  a 
few  drops  of  this  to  a  solution  of  plain  hypo  until  the  latter 
is  yellow.  This  reducer  will  not  keep  when  mixed  so  that 
the  ferricyanide  must  be  added  to  the  hypo  only  when  re- 
quired for  use.  It  is  especially  useful  for  clearing  negatives 
or  lantern  slides  and  is  often  used  for  local  reduction,  the 
solution  being  applied  with  a  wad  of  absorbent  cotton  to 
the  part  which  is  to  be  lightened.  Another  cutting  reducer 
is  permanganate,  which  is  supplied  under  the  name  of  the 
"Eastman  Reducer."  Permanganate,  however,  tends  to 
act  more  proportionally  on  the  highlights  and  shadows  than 
is  the  case  with  ferricyanide. 

Proportional  reducers  are  those  which  act  on  all  parts  of 
the  negative  in  proportion  to  the  amount  of  silver  present 
there.  They  thus  exactly  undo  the  action  of  development 
since  during  development  the  density  of  all  parts  of  a 
negative  increase  proportionately.  A  correctly  exposed,  but 
over-developed  negative  should,  therefore,  be  reduced  with 
a  proportional  reducer.  This  effect  is  shown  in  Fig.  84 
where  it  is  seen  that  the  contrast  of  the  negative  is  far  too 
^reat  owing  to  over-development,  and  that  by  removing 
the  same  proportion  of  the  silver  from  the  shadows,  half- 
tones and  highlights,  a  negative  of  correct  contrast  can  be 
obtained. 

88 


THE  FINISHING  OF  THE  NEGATIVE 


Fig.  84. 
Diagram  Showing  Plow  Proportional  Reducer  Acts. 

Unfortunately  there  are  no  single  reducers  which  are  ex- 
actly proportional  in  their  action  but  by  mixing  perman- 
ganate, which  is  a  slightly  cutting  reducer,  with  persulphate, 
which  is  a  flattening  reducer,  a  proportional  reducer  may  be 
obtained.  Flattening  reducers  are  required  for  negatives 
which  have  been  under-exposed  and  then  over-developed. 
In  these  cases  the  negative  is  much  too  contrasty  but  it  is 
important  not  to  remove  any  of  the  deposit  from  the 
shadows,  since  owing  to  the  under-exposure,  there  is  already 
insufficient  deposit  in  the  shadows. 

What  is  required  in  this  case  is  shown  in  Fig.  85,  where 
a  large  amount  of  deposit  is  removed  from  the  highlights, 
a  smaller  amount  from  the  halftones,  and  very  little  or 
none  from  the  shadows.  This  can  be  accomplished  by  the 
use  of  ammonium  persulphate.  Ammonium  persulphate  at- 
tacks silver  deposit  with  the  formation  of  silver  sulphate  and 
this  attack  is  increased  by  the  silver  salt  which  is  produced, 
the  rate  of  attack  increasing  as  the  attack  goes  on.  Such 
chemical  actions  are  called  "auto-catalytic,"  a  "catalyst" 
being  a  substance  which  increases  the  rate  of  a  chemical 


W////////////////////////M 


////////////////////////////y 


Fig.  85. 
Diagram  Showing  How  Flattening  Reducer  Acts. 

89 


FUNDAMENTALS  OF  PHOTOGRAPHY 


Fig.  86. 

a.  Negative  too  dense  all  over. 

b.  Result  of  using  Farmer's  Reducer. 


action  without  actually  taking  part  in  it,  and  an  auto- 
catalytic  action  being  one  in  which  the  rate  of  action  in- 
creases of  its  own  accord.  Since  the  action  of  ammonium 
persulphate  is  auto-catalytic  it  acts  most  rapidly  where  the 
greatest  amount  of  silver  is  present,  and  consequently  it 
attacks  the  highlights  far  more  energetically  than  it  attacks 
the  shadows  of  the  negative  and  is,  therefore,  suitable  for 
the  reduction  of  under-exposed,  over-developed  negatives. 
(Whether  any  silver  will  be  removed  from  the  shadows  will 
depend  on  how  long  the  reducer  is  allowed  to  act.)  Because 
it  is  auto-catalytic  in  its  action,  however,  it  is  very  likely 
to  go  too  far  and  get  out  of  control  so  that  it  is  not  by  any 
means  an  easy  reducer  to  handle,  and  it  is  not  recommended 
that  it  be  used  upon  a  valuable  negative  unless  the  user 
lias  had  considerable  experience  of  its  action. 

For  some  time  after  ammonium  persulphate  was  intro- 
duced   as    a    reducer    for   negatives   its  action    was   very 

90 


THE  FINISHING  OF  THE  NEGATIVE 


b 
Fig.  87. 

a.  Correctly  exposed  but  over-developed  negative. 

b.  Result  of  reducing  with  a  Proportional  reducer. 

91 


FUNDAMENTALS  OF  PHOTOGRAPHY 


H*"^  ■» 

3JwBIC5^B                ■   39 

^1 

Fig.  88. 

a.  Too  dense  in  highlights,  deep  shadows  not  elear. 

b.  Effect  of  the  Eastman  Reducer  on  such  a  negative. 

uncertain;  some  samples  would  reduce  silver  while  others 
would  not.  When  this  peculiarity  in  its  behavior  was  inves- 
tigated by  the  Research  Laboratory  of  the  Eastman  Kodak 
Company  the  reason  was  found  to  be  a  chemical  difference 

in  some  of  the  samples  tested. 

INTENSIFICATION 

Sometimes  we  get  negatives  which  are  too  thin  and  weak 
to  print  even  on  Contrast  Velox;  if  we  developed  them  in 
the  tray  perhaps  we  were  deceived  in  judging  the  density 

92 


THE  FINISHING  OF  THE  NEGATIVE 


b 

Fig.  89. 

a.  Negative  with  dense,  blocked  up  highlights. 

b.  Shows  that  a  Flattening  Reducer  removes  much  silver  from  the 

Highlights,  less  from  the  Halftones  and  little  or  none  from  the 
Shadows. 

and  we  under-developed  them,  or  possibly  the  subject  itself 
was  very  flatly  lighted,  as  often  happens  when  the  subject 
is  an  extremely  distant  landscape  or  a  view  across  a  large 
body  of  water,  and  from  such  negatives  we  cannot  get  a 
bright  print,  even  on  Contrast  Velox. 

Sometimes,  also,  we  may  not  have  Contrast  Velox  on 
hand  and  may  wish  to  use  Special  or  Regular  Velox.    In 

93 


FUNDAMENTALS  OF  PHOTOGRAPHY 


Original.  Strongly  Intensified. 

Fig.  90. 
Showing  Effect  of  Intensification. 


Less  Intensified. 


all  these  casts  it  is  convenient  to  have  a  means  of  increas- 
ing the  contrast  of  the  negative,  and  the  method  by  which 
this  is  done  is  the  chemical  process  commonly  called  "in- 
tensification." 

In  order  to  increase  the  contrast  we  must,  of  course,  in- 
crease all  the  separate  steps  of  density  occurring  in  the  nega- 
tive, and  not  only  must  we  increase  them  but  the  increase 
must  be  proportional  to  the  steps  already  existing;  that  is 
to  say,  we  must  multiply  them  all  by  the  same  amount  if  we 
are  to  retain  correct  gradation.  Fig.  91  shows  a  number  of 
different  steps  of  density  before  and  after  intensification, 
all  the  densities  having  been  multitiplied  by  the  same 
amount  or  increased  in  the  same  proportion. 

In  order  to  produce  this  increase  of  density  we  must 
either  deposit  some  other  material  on  the  silver,  so  as  to 
add  something  to  the  image  or  we  must  change  the  color  of 
the  image  so  as  to  make  it  more  non-actinic  and  capable  of 
stopping  more  of  the  light  which  affects  the  printing  paper. 

94 


THE  FINISHING  OF  THE  NEGATIVE 


There  are  many  different  substances  which  can  be  de- 
posited upon  the  image.  If,  for  instance,  a  negative  is 
treated  with  a  silvering  solution  suitably  adjusted, 
the  silver  will  be  deposited  on  the  image  and  will  increase 
its  density,  but  this  is  very  difficult  to  do,  and  it  is  more 
practical  to  intensify  negatives  by  depositing,  not  silver, 
but  mercury  upon  them. 

The  Eastman  Intensifier 
is  a  solution  containing  mer- 
cury which  will  be  deposited 


Original     densities 

Densities     added  /></    intensification 

Fig.  91. 
Densities  Added  By  Intensification. 


upon  the  negative  immersed  in  it,  and  since  the  deposition 
is  regular,  it  can  be  watched  and  the  gain  of  density  ob- 
served so  that  the  intensification  can  be  stopped  at  the 
right  time. 

While  the  mercury  method  is  still  the  most  popular  for 
intensifying  negatives  it  has  never  been  wholly  satisfactory, 
because  mercury  intensified  negatives  are  apt  to  undergo 
changes  that  affect  their  quality  after  a  time. 

Another  method  of  intensifying  a  negative  is  to  bleach 
it  in  the  Velox  re-developer  and  then  re-develop  the 
bleached  image  with  the  sulphide  solution  used  for  obtain- 
ing sepia-toned  prints.  By  this  method  the  image  is  changed 
from  silver  to  silver  sulphide,  which  has  a'  brownish-yellow 
color  and  is  much  more  opaque  to  actinic  light  than  the  or- 
iginal silver  image,  so  that  a  negative  treated  in  this  way 
will  show  much  more  contrast  than  before  treatment.  This 
method  has  proved  very  satisfactory  and  it  is  believed  that 
re-developed  negatives  will  prove  as  permanent  as  re-devel- 
oped Velox  prints. 

It  must  be  understood  that  intensification  is  only  suitable 
for  the  increase  of  contrast,  it  cannot  improve  a  negative 
which  is   seriously  under-exposed;  no  amount  of   intensi- 

95 


FUNDAMENTALS  OF  PHOTOGRAPHY 

fication  can  introduce  detail  which  is  not  present  before  the 
intensification  is  commenced ;  but  occasionally  intensifica- 
tion will  enable  us  to  adjust  the  scale  of  contrast  of  a  nega- 
tive so  that  better  prints  can  be  obtained  than  are  possible 
without  the  intensifying  treatment. 


96 


CHAPTER  X. 


^^ 

-tfBBHy8^ 

s* :- 

LiA" 

J^ 

Fig.  92. 
Halation  in  Print. 


HALATION. 

Sometimes  in  a  photograph  there  appears  to  be  a  blur- 
ring of  the  bright  parts  over  the  dark  parts  of  the  picture, 
and  if  lamps  or  other  very  bright  lights  are  included  they 

may  appear  in  the  print 
as  bright  spots  sur- 
rounded by  a  dark  ring 
beyond  which  is  an- 
other bright  ring.  This 
curious  effect,  which  is 
called  "halation"  is  well 
illustrated  in  the  photo- 
graph shown  in  Fig. 
92. 

Halation  is  caused 
by  light  which  passes 
completely  through  the 
emulsion  and  also 
through  the  glass  on 
which  the  emulsion  is  coated  and  is  then  reflected  back 
into  the  emulsion  from  the  back  of  the  glass.  The  sim- 
plest form  of  such  reflection  is  shown  by  the  diagram, 
Fig.  93,  where  we  see  a  ray  of  light  falling  on  the 
emulsion  at  A.  Most  of  this  light  is  absorbed  by  the  emul- 
sion but  some  of  it  passes  through  to  the  glass  and  is  reflect- 
ed from  the  back  of  the  glass,  so  that  it  reaches  the 
emulsion  again  at  B. 

But  this  simple  dia- 
gram does  not  account 
for  the  appearance  of 
the  lights  in  Fig.  92, 
because  if  a  ray  of  light 
had  fallen  on  the  plate 
squarely  at  right  angles 
and  had  passed  through 
the  emulsion  at  right 
angles  it  would  be  re- 
flected straight  back  and  the  halation  would  not  be  spread 
beyond  the  image,  whereas,  the  halation  is  just  as  bad  in 
the  center  of  the  picture  where  the  light  fell  squarely  on  the 

97 


Fig.  93. 
Simplest  Form  of  Reflection. 


FUNDAMENTALS  OF  PHOTOGRAPHY 


Silver  Bforrnde 

Craws 
in 

Emulsion 


Glass 

Fig.  94. 
Scattered  Reflections. 


emulsion  as  at  the  edges.    Also,  it  does  not  account  for  the 

ring  which  is  shown  around  the  lights. 

As  a  matter  of  fact,  light  falling  on  a  photographic  plate 

does  not  go  straight  through  in  this  simple  way.    When 

a  narrow  ray  of  light 
falls  on  the  grains  of 
silver  bromide  it  is  re- 
flected from  them  and 
scattered  about. 

So  we  must  imagine 
that  if  we  could  ex- 
amine a  magnified  sec- 
tion through  the  plate, 
we  should  see  the  light 
falling  on  the  emulsion 
scattered  in  all  direc- 
tions, so  that  a  narrow 
beam  of  light  is  spread 
out  into  a  kind  of  blur, 
the  size  of  the  blurring 
being  very  minute  but 
still  appreciable,  Fig. 
95;    this  effect    of   the 

light  spreading  in   the  film  is  called  irradiation. 

We  see   then    that   the  light  which   passes  through  the 

emulsion   of  the  photographic  plate    is    traveling   in   all 

directions,  whatever  may  have  been  its  direction  before 

it    reached    the  emul- 
sion, and  if   we  follow 

the  light  into  the  glass, 

we  shall  find  that  most 

of  the  rays  pass  out  ol 

the  glass  again  into  the 

air  but    that   some   of 

them  are  reflected  back 

into  the  emulsion. 

In    order    to    under- 
stand this  we  must  look 

at    the   way   in    which 

different    rays  of  light 

travel     through     glass. 

(See  chapter  III.) 

When    a    ray   of   light 

passes  from  air  into  a 

block  of  glass,  it  is  bent 

98 


Emulsion 


Glass 

Fig.  95. 
Irradiation. 


HALATION 


Front  Surface  of 


Back  Surface  of  Glass 


Fig.  96. 
Rays  in  a  Block  of  Glass. 


by  the  glass  which  is  a 
medium  of  different 
density,  and  when  it 
leaves  the  glass  again 
it  is  bent  back  so  as  to 
travel  along  a  path 
parallel  to  that  along 
which  it  entered  the 
glass,  but  if  a  ray  leav- 
ing the  glass  meets  the 


Fig.  97. 
Rays  in  Photographic  Plate. 


surface  at  too  big  an  angle,  it  cannot  go  out  and  it  will 
be  totally  reflected  back  again.     See  Fig.  96.    It  is  these 
totally    reflected    rays 
which  produce  the  ring 
of  halation. 

When  the  image  of 
the  lamp  falls  on  the 
emulsion  and  enters  it, 
the  rays  are  spread  out 
by  irradiation,  so  that 
we  get  a  small  spot  at 
the  center  of  the  lamp, 
then  this  scattered  light 
passes  into  the  glass  of 
the  plate,  and  the  rays 
which  are  near  the  cen- 
ter pass  out  into  the  air  from  the  glass  and  we  get  a  dark 
ring,  but  when  suddenly  the  angle  of  the  rays  to  the  surface 
of  the  glass  gets  too  big  to  get  out  they  are  reflected  back 
and  produce  a  sharp  ring  of  halation  around  the  center  of 
the  image,  and   then  as  they  go  farther  and  farther  from 

the  image  the  light  gets 
weaker  and  the  hala- 
tion fades  away  again. 
Thus  we  can  account 
completely  for  the  rings 
of  light  shown  in  the 
picture. 

If  we  coat  the  back 

of  the  glass  with  some 

substance    into    which 

the  rays  would  pass  di- 

Pig  og  rectly    from    the   glass 

Complete  Diagram  of  Halation.  and  which  would  COm- 


99 


FUNDAMENTALS  OF  PHOTOGRAPHY 

pletely  absorb  them,  we  should  wholly  prevent  the  hala- 
tion and  if  we  choose  this  "backing",  as  it  is  called,  so  that 
it  is  of  the  right  kind  and  almost  completely  absorbs  the 

light,  allowing  very  lit- 
tle of  it  to  be  reflected, 
then  it  will  be  quite 
effective  in  reducing 
halation,  but  in  prac- 
tice it  is  not  altogether 
easy  to  get  a  satisfac- 
tory backing  and  to 
apply  it  correctly.  The 
photographer  tried  a 
"backed"  plate,  but  al- 
though he  got  rid  of  the 
sharp  rings  of  halation 
his  lights  are  still  ob- 


Fig.  99. 
Print  from  Backed  Plate  Negative. 


scured  by  irregular  blotches  of    light   reflected    from    the 
back  of  the  glass.     (Fig.  99). 

The  best  way  of  avoiding  halation  is  not  to  have  any 
glass  at  all.  If  we  take  the  photograph  on  film,  the  sup- 
port is  so  thin  that  the  light  has  very  little  room  to  spread 
and  we  get  only  a  very  small  spreading  of  the  light  rays. 
This  spreading  in  fact  is  no  greater  than  that  necessary 
to  give  a  correct  representation  of  the  effect  of  the 
light  on  the  eye  since 
there  really  is  a  spread- 
ing of  the  light  in  the 
eye  and  we  do  not  ac- 
tually see  a  bright  light 
on  a  dark  night  as  per- 
fectly sharp,  but  as 
having  a  small  amount 
of  blur  around  it.  So 
that  in  Fig.  100,  which 
was  taken  on  Kodak 
film,  we  get  a  result 
which  gives  a  very  good 
idea  of  the  scene  as  it 
appeared. 


Fig.  100. 
Print  From  Negative  Taken  on  Film. 


I  CO 


CHAPTER  XI. 

ORTHOCHROMATIC  PHOTOGRAPHY. 

IF  we  take  a  piece  of  blue  cloth  and  put  an  orange  on  it 
and  then  photograph  the  combination  we  shall  find  that 
instead  of  the  orange  being  lighter  than  the  cloth,  as  it  looks 
to  the  eye,  the  photograph  (Fig.  101)  shows  it  as  being  dark- 
er. This  difficulty  in  photographing  colored  objects  so  that 
they  appear  in  the  print  in  their  correct  tone  values,  as  they 
are  seen  by  the  eye,  has  been  well  known  to  photographers 
from  the  earliest  days  of  the  art. 

In  order  to  understand  the  cause  of  it  we  must  consider 
the  nature  of  color  itself.    When  we  speak  of  a  colored  object 


Fig.  101. 
Picture  of  an  Orange  on  Blue  Cloth. 


IOI 


FUNDAMENTALS  OF  PHOTOGRAPHY 

we  mean  one  which  produces  a  distinct  sensation,  which  we 
call  the  sensation  of  color.  This,  of  course,  is  due  to  a 
change  in  the  nature  of  the  light  which  enters  the  eye  and 
causes  the  sensation  of  sight,  and  this  change  is  produced 
in  the  light  by  the  colored  object  so  that  the  light  after 
reflection  from  the  colored  object  is  different  in  composition 
from  the  beam  of  light  before  reflection. 

In  Chapter  II  we  have  seen  that  light  consists  of  waves, 
and  that  these  waves  are  of  various  lengths,  the  color  of 
the  light  depending  upon  the  wavelength. 


BLUE  VI 


OLET 


GRiiEN 


ORA  NEE 
YELLOW 


RED 


400 


450 


500 


550 


600 


700 


Fig.  102. 
Divisions  of  Spectrum. 


In  white  light  there  are  waves  of  all  lengths  and  if  white 
light  is  passed  through  a  spectroscope  it  is  spread  out  into 
a  band  of  various  colors  which  is  called  a  spectrum.  The 
various  colors  of  the  spectrum  correspond  to  definite 
lengths  of  light  waves  and  if  we  measure  their  length  in 
the  very  small  units  which  are  used  for  measuring  waves 
of  light  we  shall  find  that  the  red  waves  are  700  million ths 
of  a  millimeter,  the  yellow  ones  are  600,  the  green  550,  the 
blue-green  500,  the  blue  450,  and  the  violet  waves,  the 
shortest  which  we  can  see,  are  400  millionths  ot  a  milli- 
meter long  (Fig.  102).  Thus,  we  can  scale  the  spectrum 
by  the  length  of  the  light  waves  of  which  it  is  composed 
(Fig.  102). 


Fig.  103. 
Pink  filter  passing  violet,  blue,  yellow,  orange  and  red  rays  but 
absorbing  green. 

If  we  take  a  piece  of  colored  glass  or  gelatine,  say  pink 
gelatine,  and  hold  it  in  front  of  the  spectrum,  we  shall  find 
that   the  pink  gelatine  will   not   let  some  of   the  waves  of 

I02 


ORTHOCHROMATIC  PHOTOGRAPHY 

light  through;  it  will  stop  them  completely,  while  it  will 
let  the  other  waves  through  without  any  difficulty.  The 
pink  gelatine,  in  fact,  cuts  out  or  absorbs  the  green  light 
(Fig.  103).  This  is  because  of  its  pinkness;  that  is,  it  has 
the  property  of  absorbing  green  light  from  the  white  light 
and  of  letting  through  the  other  light  which  is  not  green, 
that  is  to  say,  to  a  less  degree  this  pink  film  sorts  out  the 
light  just  as  the  spectroscope  does,  but  instead  of  separating 
the  waves  of  different  lengths  it  stops  some  of  them  and 
lets  the  others  go  on,  and  the  eye,  missing  those  which  are 
stopped,  records  the  absence  as  a  sensation  of  color. 

If,  instead  of  having  a  transparent  substance  like  film, 
we  have  an  opaque  colored  object,  like  a  sheet  of  orange 
paper,  and  let  the  spectrum  fall  on  it,  we  shall  find  that  the 


Fig.  104. 

Purple  filter  passing  violet  and  red,  but  absorbing  the  blue,  green, 

yellow  and  orange. 

orange  paper  will  reflect  the  red  and  yellow  and  green  light 
but  will  refuse  to  reflect  the  blue  light;  it  absorbs  it,  and  its 
orangeness  is  due  to  the  fact  that  it  absorbs  the  blue  light 
and  refuses  to  reflect  it.  All  objects  which  are  colored 
are  colored  because  they  have  some  selective  absorption  for 
some  of  the  waves  of  light;  they  do  not  treat  them  all  alike 
but  reflect  some  and  absorb  others,  and  the  modified  light 
which  reaches  the  eye  we  call  "color."  Any  object  which 
treats  all  the  waves  of  light  alike,  which  absorbs  them  all 
or  absorbs  them  equally  or  reflects  them  all  in  equal  pro- 
portion, is  not  colored.    If  it  absorbs  them  all  it  will  be 


Invisible  Limit  of    Violet 

Ullra-Violet  Visibility 


Deep- 
Red 


Limit  of 
Visibility 


IO3 


FUNDAMENTALS  OF  PHOTOGRAPHY 

dead  black  since  it  will  reflect  no  light.  If  it  absorbs  them 
to  a  small  extent,  but  equally,  it  will  be  gray;  if  it  reflects 
them  all  it  will  be  white,  but  if  it  absorbs  some  of  the  wave 
lengths  and  not  others,  it  will  be  colored. 

If  we  try  a  series  of  experiments  in  our  spectrum  we  shall 
find  that  things  which  absorb  red  light  are  colored  blue, 
and  those  which  absorb  green  light  are  colored  pink  or 
magenta,  or  if  they  absorb  a  great  deal  of  the  light,  purple 
(Fig.  104).  Those  that  absorb  blue-green  light  are  orange, 
and  those  that  absorb  blue-violet  light  are  yellow.  We  see, 
then,  that  to  each  color  there  corresponds  a  region  of  the 
spectrum  which  is  absorbed. 

If  we  look  at  a  spectrum  we  shall  see  that  the  brightest 
part  of  it  is  the  yellow-green  and  yellow  (the  position  of 
the  yellow  in  the  spectrum  being  between  the  yellow-green 
and  the  orange)  so  that  the  eye  is  most  sensitive  to  the 
yellow,  yellow-green  and  red  rays  and  least  sensitive  to  the 
blue  and  violet  rays.  (Fig.  105.)  But  if,  instead  of  looking 
at  the  spectrum,  we  use  a  piece  of  bromide  paper  so  that  the 


Invisible  Limit  of   Violet    Blue     Blue-    Green      Yellow-  Orange    Red     Deep-     Limit  of 

Ultra-Violet  Visibility  Green  Green  Red      Visibility 

Fig.  106. 

light  of  the  spectrum  may  fall  on  it,  and  then  make  a  posi- 
tive print  from  this  negative  image,  we  shall  find  that  the 
photographic  action  on  the  print  is  not  produced  in  the 
region  that  is  bright  to  the  eye,  but  in  the  region  which  the 
eye  can  scarcely  see,  and,  indeed,  there  is  a  strong  action 
in  the  part  of  the  spectrum  beyond  the  visible  spectrum, 
showing  that  there  are  waves  which  are  shorter  than  the 
violet  waxes,  which  were  discovered  when  the  spectrum 
was  first  photographed  and  are  called  the  ultra-violet  waves. 
(Fig.  106.)  This  explains  at  once  why  when  we  photograph- 
ed ,m  orange  on  a  blue  cloth  the  orange  was  dark  in  the 
photograph  and  the  blue  cloth  was  bright,  which  is  the  op- 
posite to  the  way  they  appear  to  the  eye.  The  bright  orange 
absorbs  the  blue  light  to  which  the  film  is  sensitive  and  the 

104 


ORTHOCHROMATIC  PHOTOGRAPHY 

blue  cloth  reflects  It,  so  that  although  the  cloth  looks  dark 
to  the  eye,  it  is  bright  in  the  photograph,  and  the  orange 
which  reflects  very  little  blue  and  violet  light  is  dark  in  the 
photograph.  Fortunately,  this  defect,  for  defect  it  is,  of 
photographic  materials  can  be  remedied  to  a  considerable 
extent. 

If  dyes  are  incorporated  with  the  emulsion  the  dyes  sensi- 
tize the  emulsion  for  the  part  of  the  spectrum  which  they 
absorb,  so  that  if  we  put  a  pink  dye  of  the  right  kind  in  the 


Invisible     Limit  of    Violet    Blue    Blue-    Green    Yellow-    Orange     Red    Deep-   Limit  of 
Ultra-Violet  Visibility  Green  Green  Red       Visibility 

Fig.  107. 

emulsion  the  film  will  not  only  be  sensitive  to  the  blue  light, 
to  which  it  is  naturally  sensitive,  but  will  also  become 
sensitive  to  the  yellow-green  light,  which  the  pink  dye 
absorbs,  and  if  we  take  a  photograph  of  the  spectrum  on 
this  sensitized  film  we  shall  get  a  photograph  which  appears 
as  is  shown  in  Fig.  107.  Film  made  sensitive  in  this  way  is 
called  ortho chromatic,  and  in  photographing  colored  objects 
the  use  of  an  orthochromatic  film  is  a  great  advantage. 

The  orthochromatic  film  is  still  not  sensitive  to  red,  which 
to  the  eye  is  a  bright  color,  and  so  red  objects  are  still  too 
dark  when  in  a  photograph,  but  this  is  not  a  great  disad- 
vantage for  most  work,  and  we  have  the  very  great  ad- 
vantage that  the  film  can  be  developed  in  a  red  light. 

It  is  possible  to  treat  a  film  with  dyes  which  make  it 
panchromatic,  that  is,  sensitive  to  all  colors,  but  a  panchro- 
matic film  would  have  to  be  made  and  developed  in  total 
darkness,  and  that  is  so  difficult  that  it  is  better  to  be 
content  for  most  work  with  the  orthochromatic  film,  which, 
when  properly  handled,  enables  a  good  rendering  of  most 
colored  objects  to  be  obtained  and  at  the  same  time  is  easy 
to  use. 

Great  care  is  taken  to  make  Eastman  NC  film  as  ortho- 
chromatic as  will  confer  satisfactory  color  sensitiveness  upon 
it  without  sensitizing  it  so  far  that  it  will  be  difficult  for  the 

105 


FUNDAMENTALS  OF  PHOTOGRAPHY 


Fig.  108. 
Made  Through  a  Yellow  Light  Filter. 

user  to  handle  or  that  there  will  be  danger  of  fog  when 
developing  it. 

While  the  sensitizing  with  dye  makes  the  film  sensitive 
to  the  yellow  and  green  light,  it  is  still  much  more  sensitive 
to  the  blue  and  violet  waves,  as  is  shown  in  Fig.  107,  and 
consequently  it  will  still  photograph  blue  objects  much 
lighter  than  they  appear  to  the  eye.  This  is  a  disadvantage 
in  some  photography,  and  especially  in  landscape  photo- 
graphy where  we  have  blue  sky  with  white  clouds.  White 
clouds  are  much  brighter  to  the  eye  than  the  blue  sky,  but 
if  they  are  photographed  on  the  film  in  the  ordinary  way 
the  blue  sky  appears  too  light  and  the  clouds  are  lost  against 
it.  In  order  to  overcome  this  and  to  enable  orthochromatic 
film  to  represent  most  of  the  colors  in  their  correct  tone 
values  light  filters  are  used  which  absorb  the  excess  of  blue 
light  and  prevent  it  from  reaching  the  film. 

These  light  filters  are,  of  course,  yellow  in  color,  since 
yellow  absorbs  blue  light  and  thus,  by  the  use  of  yellow 
light  filters,  which  are  sometimes  called  color  screens,  the 
excess  of  blue  light  can  be  absorbed  and  a  much  improved 
rendering  of  sky  and  clouds  can  be  obtained.    (Fig.  108.) 

When  light  filters  were  first  introduced  it  was  thought 
that  any  yellow  glass  would  be  satisfactory,  and  light  filters 
were  made  of  brownish  yellow  glass,  which  really  are  of  no 
advantage  at  all.  The  reason  for  this  is  that  they  transmit 
the  ultra-violet  light,  which  lies  out  in  the  spectrum  beyond 

106 


ORTHOCHROMATIC  PHOTOGRAPHY 

the  violet.  This  ultra-violet  light  is  quite  invisible,  but  pro- 
duces a  strong  impression  upon  the  photographic  plate,  and 
in  order  to  get  satisfactory  action  from  a  filter  it  is  very  im- 
portant to  remove  the  ultra-violet  light  as  completely  as 
possible.  The  ultra-violet  light  is  far  more  easily  scattered 
by  traces  of  mist  in  the  atmosphere  than  visible  light,  and 
since  it  is  this  mist  which  so  often  makes  objects  in  the  dis- 
tance invisible  in  photographs  that  are  taken  without  a 
filter  (Fig.  Ilia)  it  is  necessary  to  use  a  filter  that  will  cut 
out  this  ultra-violet  light  in  order  to  show  the  distance 
well.  (Fig.  111b.) 

Modern  light  filters  are  made  by  dyeing  gelatine  with 
carefully  chosen  dyes  and  then  cementing  the  dyed  gelatine 
between  optically  prepared  glasses. 

Some  yellow  dyes,  while  removing  violet  light  quite  sat- 
isfactorily, transmit  a  great  deal  of  the  ultra-violet  light 
and  only  a  few  dyes  cut  out  the  invisible  ultra-violet  satis- 
factorily.   One  of  the  best  of  these  dyes  is  the  dye  used  in 


INVISIBLE  ULTRA-VIOLET  /       BLUE  GREEN  RED 

LIMIT  OF 

VISIBILITY 

Fig.  109 

Photograph  of  the  spectrum,  through  two  yellow  Filters,  which  are  of  almost 

the  same  color  to  the  eye,  showing  (A)  that  the  K  Filter  cuts  out 

the  ultra-violet,  while  (B)  the  other  Filter  does  not. 

the  Wratten  K  filters  and  the  Kodak  Color  Filters.  In 
Fig.  109  are  shown  two  photographs  of  the  spectrum — the 
one  taken  through  a  filter  made  with  a  dye  of  a  type  often 
used  for  filters,  but  not  cutting  out  the  ultra-violet,  and  the 
other  the  same  spectrum  taken  through  a  K  filter. 

The  K  filters  were  made  with  a  dye  produced  in  Germany, 
and  during  the  war  the  requirements  of  the  aerial  photo- 
graphers in  the  army  made  it  necessary  to  prepare  a  new 
dye  which  could  be  made  in  America  and  which  would  cut 
the  mist  even  more  sharply  than  the  K  filters.  This  pre- 
sented a  problem  which  was  solved  in  the  Kodak  Research 
Laboratory  by  the  discovery  of  an  entirely  new  dye  which 
was  named  "Eastman  Yellow,"  with  which  special  filters 
are  prepared  for  aerial  photography. 

107 


FUNDAMENTALS  OF  PHOTOGRAPHY 

Since  a  yellow  light  filter  removes  the  ultra-violet  and 
much  of  the  blue-violet  light,  it  necessarily  increases  the 
exposure,  because  if  we  remove  those  rays  to  which  the 
film  is  most  sensitive,  we  must  compensate  for  it  by  expos- 
ing the  film  for  a  longer  time  to  the  action  of  the  remaining 
rays,  and  the  amount  of  this  increased  exposure  will  be  de- 
pendent both  on  the  proportion  of  the  violet  and  the  blue 
rays  which  are  removed  by  the  filter  and  also  on  the  sensi- 
tiveness of  the  film  for  the  remaining  rays  (green,  orange 
and  red)  which  are  not  removed  by  the  filter. 

The  number  of  times  by  which  the  exposure  must  be  in- 
creased for  a  given  filter  with  a  given  film  is  called  the 
"multiplying  factor"  of  the  filter,  and  since  the  factor  de- 
pends both  upon  the  depth  of  the  filter  and  upon  the  color 
sensitiveness  of  the  film,  it  is  meaningless  to  refer  to  filters 
as  "three  times"  or  "six  times"  filters  without  specifying 
with  what  material  they  are  to  be  used. 

It  is  always  desirable  that  we  should  be  able  to  give  as 
short  an  exposure  as  possible;  what  is  required  in  a  filter  is 
that  it  should  produce  the  greatest  possible  effect  with  the 
least  possible  increase  of  exposure,  so  that  a  filter  will  be 
considered  most  efficient  when  it  produces  the  maximum 
result  with  the  minimum  multiplying  factor.  To  a  certain 
extent  the  multiplying  factor  depends  upon  the  result  that 
is  wanted;  thus  in  order  to  get  exactly  the  same  proportional 
exposure  when  using  a  Kodak  Color  Filter  with  Eastman 
NC  Film,  as  that  obtained  without  it  the  necessary  in- 
crease of  exposure  is  ten  times, 
but  in  fact  the  Color  Filter  is 
generally  used  for  distant  land- 
scapes where  haze  is  to  be  cut 
out,  and  for  clouds  against  the 
sky,  and  under  such  conditions 
an  increase  of  three  times  the 
normal  exposure  that  would  be 
correct  for  an  ordinary  land- 
scape will  give  the  most  satis- 
factory results. 

For  many  purposes,  however, 
Fig.  IK'-  the   Kodak   Color    Filter   is  too 

Kodak  sky  Filter.  strong;  the  exposure  when  usin.u 

it  is  so  prolonged  that  it  is  not 
practical  to  use  the  Kodak  without  a  tripod,  and  to  meet 
these  difficulties  the  Kodak  Sky  Filter  has  been  intro- 
duced.   (Fig.  110.) 

108 


ORTHOCHROMATIC  PHOTOGRAPHY 


b 
Fig.  111. 
Made  without  a  filter. 
Made  with  a  Wratten  G  filter. 


In  this  filter  only  half  the  gelatine,  which  is  cemented 
between  the  glasses,  is  stained  with  the  yellow  dye,  the 
other  half  being  clear,  and  the  filter  is  placed  on  the  lens 
with  its  stained  half  on  top  so  that  the  light  from  the  sky 
will  pass  through  the  stained  half  and  the  light  from  the 

109 


FUNDAMENTALS  OF  PHOTOGRAPHY 

landscape  through  the  clear  half  of  the  filter.  In  this  way 
the  yellow  dye  reduces  the  density  of  the  sky  in  the  nega- 
tive without  greatly  affecting  the  exposure  of  the  fore- 
ground and  enables  us  to  get  a  rendering  of  clouds  in  a 
blue  sky  by  cutting  out  a  part  of  the  very  strong  light  that 
comes  from  the  sky,  while  the  exposure  necessary  is  in- 
creased only  to  a  small  extent. 

The  sky  filter  is  not  suitable  for  the  cutting  of  haze  since 
its  colored  half  does  not  cover  the  landcsape,  which  is  the 
part  of  the  field  where  the  haze  occurs.  Its  use  is  confined 
to  that  suggested  by  its  name. 

When  it  is  desired  to  make  blue  photograph  somewhat 
darker  than  can  be  done  with  the  Kodak  Color  Filter  the 
Wratten  K2  should  be  used,  and  for  recording  still  more 
contrast,  which  is  sometimes  wanted  in  pictures  of  extremely 
distant  landscapes  that  are  under  haze,  the  Wratten  G 
filter  is  very  valuable.  Thus,  distant  mountains  and  all 
other  distant  landscape  scenes  (Figs.  Ilia  and  111b)  may 
be  photographed  through  a  strong  yellow  filter  by  giving 
the  necessary  increase  of  exposure,  with  a  Kodak  mounted 
on  a  tripod.  The  K2  will  require  an  increase  of  exposure  of 
about  twenty  times  and  the  G  of  one  hundred  times  on  the 
Kodak  Film. 


a  b 

Fig.  112. 

a.  Original  Definition. 

b.  Definition  after  screwing  up  tightly  in  cell. 


In  order  that  filters  may  not  spoil  the  definition  it  is  im- 
portant that  the  glasses  between  which  tiny  are  cemented 
should  be  of  good  optical  quality.  This  is  very  carefully 
controlled  in  the  case  of  the  Kodak  and  Wratten  filters, 


no 


ORTHOCHROMATIC  PHOTOGRAPHY 

which  are  all  measured  by  an  instrument  specially  built  for 
the  detection  of  optical  errors  introduced  by  filters.  The 
filters  have  to  be  mounted  in  the  cells  so  that  they  cannot  be 
strained  by  pressure  being  put  upon  them,  since  if  they  are 
squeezed  the  balsam  with  which  they  are  cemented  to- 
gether will  be  displaced  and  the  definition  will  be  spoiled. 
(Fig.  112.) 

Filters  should  be  treated  with  care  equal  to  that  accorded 
to  lenses.  When  not  in  use  they  should  be  kept  in  their 
cases  and  on  no  account  allowed  to  get  damp  or  dirty. 
With  reasonable  care  in  handling  they  should  never  become 
so  dirty  as  to  require  other  cleaning  than  can  be  given  by 
breathing  upon  them  and  polishing  with  a  clean,  soft  piece 
of  linen  or  cotton  cloth.  A  filter  should  never  be  allowed  to 
become  wet  under  any  circumstances,  because  if  water 
comes  into  contact  with  the  gelatine  at  the  edges  of  the 
filters  it  will  cause  the  gelatine  to  swell  and  so  separate  the 
glasses,  causing  air  to  run  in  between  it  and  the  glass. 

The  dyes  used  for  filters  are  quite  stable  to  light,  and  no 
fear  of  fading  need  be  felt.  The  filters,  however,  should  be 
kept  in  their  cases  when  not  in  use  in  order  to  protect  them. 


FINIS 


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